You can have as many sites and domains as you want, as long as you don’t go over your space limit. That’s your space: what you do with it is up to you, as long as it doesn’t violate the terms of service, and you don’t go outrageously over your limit. Email account space also contributes to your space limitations.
Basically: you can pile as many chairs as you want into your empty apartment, but sooner or later, you’re going to run out of space. Unless you have Time Lord technology. In which case, you need to contact me immediately! 🙂