I know this is a vague question. But here goes nothing,
Assuming i have a P4 Hyperthreading 3.0 Ghz, 1Gig of ram. Running Mysq/Php/apache!, how many wordpress installation do you think i can fit in there assuming they are all doing normal blogging load? (i donno, some thousand views a day) Oh yeahh, no heavy plugins, and this machine WOULD NOT be dealing with any media, vid etc?
Answer 1: loads
Answer 2: no idea
Bottom line is that you could have loads of WP blogs on a dedicated server. How many can be efficiently served is, I guess, something you'll be finding out? :)
it would largely depend on how efficiently the server is configured. a badly configured server would not be able to handle a single wordpress site as i have seen in the past...
and if your blogs get popular, u would be targeted by comment spam networks that makes your work even more complicated. i just shifted from a celeron based machine to a pentium d machine coz the attacks were making my life a whole lot more miserable.
Geeeeeeeeeesh, i hate spammers, i hate spammers SOOOOOO MUCH. There you are ACTUALLY spending an extra 50 bucks a month because of some d1ckl3ss loony that wants EVERYBODY to buy ch1nese viagra!