You can have as many email (POP/IMAP/Webmail) accounts as allowed by your service plan. If you control multiple domains, the total number of email accounts is tallied per-master-account, not per-domain. Setting up email accounts should be the first thing you do once your Birdhouse account is activated. Doing so is fairly straightforward.
Visit your cPanel, select Mail Management | “Add/Remove Accounts.” Click “Add Account” and enter an address, password, and disk quota for the account. You can also use this interface to change quotas or passwords, or to remove an account. A good mail quota for an average user is 30-50 MBs, but you can set the quota to any size permitted by your service plan.
Since you can have an unlimited number of forwarders, it may not always be necessary to create a new mailbox just to use a new email address.