One subscription = one mailbox but you can have multiple email addresses per mailbox. If you need to be able to send outbound as those addresses, there are some workarounds using distribution groups.
You can also, at no charge, create additional "shared mailboxes" which do not require a license. A shared mailbox needs to be accessed by a user that has a paid license though, it shows up as an additional mailbox in your Outlook profile. Shared mailboxes are also 50 GB in size now (previously 10 GB) and you can configure yourself with access to send outbound as those mailboxes if desired.