The number of mailboxes that you can have depends on which web hosting plan you have.
For hosting plan Start, they are 50, for hosting plan Pro they are 200, for hosting plan Biz they are 500. Hosting plan Elite offers unlimited number of accounts.
The maximum number of mailboxes (e-mail accounts) applies to all hosting plans, not for the each hosted domain in your account.
Created by you mail forwards are also included in this number.