All Pobox accounts have access to select addresses at a number of Pobox domains, including pobox.com, onepost.net, foobox.com and more. Pobox accounts may also include addresses at MyPobox personal domains, at any domain you (or another Pobox user who has invited you to use addresses at their domain) have hosted at Pobox. These addresses are all aliases for your delivery addresses (whether they're forwarding addresses or a Mailstore Inbox or a combination of both.)
Including addresses at Pobox domains and personal domains, the alias limit for each account type is:
Pobox Basic: 20 (3 @ Pobox domains)
Pobox Plus: 40 (6 @ Pobox domains)
Mailstore: 100 (6 @ Pobox domains)