One thing I've noticed keeps popping up in discussions is to take two of big dinosaurs. Informal analysis suggests two Bastiladons is very effective, one Bastiladon is marginal. Carnosaurs are considered below a Slann in utility but two Carnosaur lists have been proved to be quite potent. My own experience as well as looking at other's writing suggests taking two Ancient Stegadons is a useful addition to many armies while taking one is highly risky. I find one Razordon doesn't put out the damage output I'd hope for but two usually gets the job of clearing chaff done. Taking two Salamanders seems to beat one because your odds of one of the guys getting a nice juicy hit goes up and you can move them up separate flanks. To some extant I see people talk up two cowboys over one. I have a sneaking suspicion that TWO Troglodons would actually work in an army with lots of PF units and a WD Slann. I noticed WOC players tend to take two Hellcannons or zero. Two units of Skullcrushers or zero. Not one of each. VC players seem to take two Terrorgheists or two Mortis Engines. Not one of each. Do Monsters and other heavies universally work better in pairs or does it just SEEM that way because if a player's favorite unit is X they will take two of it? Has anyone tried a menagerie of ones. One Carnosaur, one Bastiladon, one Ancient Stegadon, etc? I'm eager to read what others think?
I did take a Scarnosaur, Solardon, and Ancient with EotG in a T&T game once. Granted, there were 5 1500ish point armies on the table. Also, I held them in reserves until about turn 3 (really only had my giant unit of Saurus Warriors on the table beforehand). They showed up, I used T&T cards/bribes to protect them, then they triple charged an Ogre Death Star and obliterated it. The I buff, ward save, Terror, impact hits, sharpened wounds...it was pretty brutal. Plus, my Lustria.com dice rolled very well for impact hits and D3 wounds. I was able to keep them in combat through overruns, pursuit, etc the whole remainder of the game. Not really a standard example in the least bit. Now, I have two Ancient Stegs, and have always taken both ever since I finished painting the 2nd. I am looking forward to the second Carnosaur and Basti. Maybe one of the guys who runs lots of PF lists of Saurus/Krox could try a double Troglodon? Sleboda maybe?
I go by something I once learned by a well known Starcraft 2 gamer and commentator. Don't ever bring too much (units), focus on a few things and make those few things strong. On other words make it simple. This can basically be applied to every strategy game ever released and certainly also fits within warhammer. We Could make an army fielding a singe unit of 3 Ripperdactyls, but honestly it would probably not work as intended. Making a mixed army with a little bit of this and a little bit of that will make for an overall very weak army. Picking a single steg though might not be bad because it's still a lot of points for a single model, but having a couple that works together can be truly devastating should they get off their impact hits on the same unit. I'm not sure about the troglodon though. It's still just 3 attacks at S5. If it had more attacks or the spit would've worked as a small template then maybe, but paying 400pts for two of those isn't quite worth it. You're just brining more mediocrity.
This idea is one of my favorite things to talk about in Warhammer. Yes. Yes, you should double up. Some people (very few) can do well with a little of this and a little of that. Not me. My stuff that hits on a 2+ misses half the time. My units that only fail break checks 10% of the time, do so 40%. Redundancy is key. If you build a plan around the idea that you have something you can do, call it "Capability X", then you damn well better be sure your army can do it. Don't go into battle with uncertainty if you can avoid it. It's not just doubling up. It's about weighting the odds. So, if you are planning to have your Casket of Souls kill things with its spell, take Death Magic and make sure you reduce your opponent's Ld. If you plan for your Stegadon impact hits to make a dent, take 2 of them so you don't fluff the number of hits. Whatever you want your army to do, do it! Don't pussyfoot around or one bad roll can eff you up. This can lead to some armies that lack variety and have a harder time dealing with specific opposition, but so what? You will be focused on _your_ plan and get better at it, rather than trying to anticipate everything that every army might possibly do to you. If you get your butt kicked by the rock to your paper, build your next list to be scissors. You're going to play a ton of games. Get your army list variety in over the course of all your games instead of in just one game.
Can't redundancy be accomplished while using different units? For instance, instead of having two units of 8 Chameleon Skinks for war machine hunting, or two units of 3 Terradon Riders, you could have one of each. While not identical, they do serve some of the same purposes and it seems like the variety would provide a benefit over doubling up on one or the other. An opponent can't stymie two units of Chameleon Skinks with their deployment if you are running one of each, for example. I could see a similar approach working with your choice of hammer units, etc. This sort of goes along with the original poster's question about using lots of monsters, but a variety of them. Some of them serve overlapping purposes, so you might achieve redundancy in utility without having redundancy of actual unit types. I suppose the downside of the approach is that one of the similar but not identical units is usually going to be superior in any given situation to the other, so an opponent can eliminate that one unit and remove more than half of the threat posed by the two units jointly.
I think the reason for the "Rule of Two" is as following: When you have one and it is down before doing anything it is worthless. When taking two there is always a backup which can do the same thing. For example: "I take two ancient stegadons against Ogres. He brings a cannon and one of my stegadons got shot after his first turn. In my second turn the second ancient stegadon is into combat so the stegadons worked great. When I used one in this situation, my stegadon got shot in the first turn and I got none left. So they worked terrible, but the same happened but there was no second. " This is what i mentioned with back-up, bringing two makes you more certain in making your strategy work. However, this rule doesn't apply in every situation. I often use one ancient stegadon and one bastiladon. The bastiladon is placed between my saurus and templeguard unit to ensure a initiative bonus. A slann ensures the boost (highmagic) of initiative or debuff (WD) of the initiative at my opponent. This makes the bastiladon work perfectly against almost every enemy. This only don't work against elves, +1 or +2 initiative doesn't make sense when the enemy has 4 more initiative then you. Why does my ancient stegadon work? Well I don't really know, I think i take the risk. Sometimes he got shot in the first turn, when the opponent is lucky. Sometimes he will make it so he can charge. The bastiladon is mostly not a threat to enemies but the combination of a slann with a bastiladon works great. I think the "rule of two" applies when for example two bastiladons gives you more certainty and when the second choice improves the functionality of the first. Two stegadons makes the stegadon tactics more viable, you often gain at least one into combat. Two bastiladons ensures you of boosting the initiative to a value that makes you strike first or at the same time. Two rippers ensures you of bringing at least one into combat before getting shot. When you bring two in combat into a unit with a blot toad marker it is just a massacre. Volume of dice!!! Two carnosaurs --> read stegadon etc. I think it is all about certainty and ensuring your tactics will work.
My guess have always been "doubles of a few" increases target saturation", because taking 1 of each makes it easier to focus on a specific target. Say the basti just gave youu enough initiative to be faster, or just as fast, as the enemy. Now the basti is a viable target, because it makes you lose your edge in combat, and thus he needs it out before you get there. But you also have a stegadon, a unit significantly faster than your saurus. So he will obviously be shootig at the stegadon first - kill that, and then go for the basti afterwards. He have a clear target priority, making decisions easier to make. When you have two stegs and two bastis, it now becomes a nightmare to pick out the right target. I had a battle where I had 1 steg, 1 basti and 1 trog. the enemy dwarf took a quick glance on my table side, and then just lined up his shots to take my monsters out in the order mentioned. Second game against him, I brought 2 stegs and 2 bastis. He spend almost 5 minutes picking a target the first turn, and at least a full minute each turn afterwards. His decisions basically became "Whatever, I'll just shoot that one", because no matter what he did, he was letting me have the advantage. It is ironically the same reason WD works as well as it does - the enemy doesn't know when it the best time to throw those dispel dice, and if they have a dispel scroll, chances are he will wiff it on something completely pointless. You just don't have the super-powerful spells that would otherwise be perfect targets for those dice/scrolls.