When you visit visit https://your-site.com/postfixadmin/setup.php you'll see this -

After creating and adding the setup password hash into your config file, and then logging into the setup page with that password, you should see :

If there are any hosting errors, or issues with your environment, they may be listed here.

Create a new admin account using your setup password .... then you can login as an admin and start creating domains and mailboxes.

You can create as many mailboxes as you want ...