Each Pobox Mailstore account is provided with one inbox. Any address that forwards mail to the Mailstore is forwarding mail to the same inbox. This means if Sally@pobox.com and Joe@pobox.com were both forwarding mail to the Mailstore, they would both be sending mail to the same inbox.
If you wanted to have more than one inbox, you would need multiple Mailstore accounts. Additionally, you can add additional Mailstore accounts to your billing group and receive a discount on the total cost!