This is quite handy and I did not see a limit… I’ve set up 2 already for fun. You’d be amazed (though perhaps not) at what was already taken… I tried quite a few profanities as well as a few funny things I was thinking of, but dammit someone got firstname.lastname@example.org already.
“You can use email aliases instead of your .Mac email address to keep your .Mac account private (such as when purchasing online) or to easily recognize and organize incoming emails from various sources. Simply deactivate or delete an email alias to keep unwanted email from arriving in your Inbox. [The Cult of Mac Blog]