currently i have 4, and I am debating whether to start up another one as a showcase thread for a specific project or not.
Reaper's Forums pretty much insist on this. They like to keep their Show Off section searchable, by Name of, or SKU number of, the models in the thread.
I only have one, and I think one to three is enough. Maybe if I do more terrain I'll make an extra thread for it, and maybe I'll split my painting thread into one for Seraphon and one for other projects. More would be confusing IMO.
I have two myself: NIGHTBRINGER's Terrain & Warhammer Woodworking Projects NIGHTBRINGER'S PAINTING AND CONVERSION BLOG! OH MY! I feel that the content in the two blogs are sufficiently distinct from one another. If I tried to combine them, it would be too much of a mixed bag. On the other hand, I didn't create a separate blog for each of my armies (WoC, LM & TK), because I feel that painted models from each are better suited to share a single blog. In short, I think that the uniqueness of the content should be the determining factor when deciding on the number of blogs you create.
I got 1 for all of my Lizardmen stuff and other models and terrain, and the other one is for all the free avatars. Ps. plenty of gallery categories for showing off your Lizardmen stuff.