The question then, is what is the limit for the other platforms... here are my predictions:
Wordpress.com: 25 blogs per account.
They say they're limitless, but setting up a wordpress account is time consuming, and I think they just assume no one will set up that many accounts... 25, max.
Tumblr: 65 blogs per account.
I say this mostly because tumblr is strange and constantly changing it's platform and updating how it works... so I think a weird number like 65 makes sense.
LiveJournal: 1 blog per account.
They actually support limitless, but everyone who's on livejournal is too busy crying over themselves to start up more than one blog.
PREDICTIONS COMPLETE!