it really depends on your pricing schedule for overcharges. if you don't oversell, you could fit one client @ 2G/52G, 2 @ 1G/26G, etc. all the way to whatever limit your host has imposed. Overselling is when you sell more than that, ie 2 @ 2G/52G, etc, planning that your clients don't use all their space - just as you won't be using all your space for a while
Figure out how much overselling you feel comfortable with, add up all the xfer and HDD overcharges you might incur, and see if your budget could handle it worst-case. Also consider that your clients may go over their alloted amount, so be sure to factor that into their pricing schedule.
As far as how many in general, it really depends on the server you have and your hosts rules, etc.