We've got servers with 100 accounts on them and we've got servers with over 2000 on them. The "100 account" servers tend to be the heavier users and the 2000 account servers tend to just be "single flat HTML webpage" sites. It just depends on what sort of resources each account needs which will be the deciding factor (you could, in theory, have a server with 6000+ accounts without problems, but I've manage to overload a server with a single account).