In my case, my server is called “box.business-mail.nl”, but I’m receiving mail for “doofpot.nl”, my private domain.
Everyone communicaties to/from my private email adres (second domain) and the hostname is still box.business-mail.nl. That’s completely hidden for normal users, but only when you preview the full headers of your email, you will see de details.
Received: from authenticated-user (box.business-mail.nl [220.127.116.11]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by box.business-mail.nl (Postfix) with ESMTPSA id 7F68B2A0D1C for email@example.com; Sun, 11 Feb 2018 10:09:05 +0100 (CET)
You don’t need an extra IP for different mail domains; you also don’t need an extra ip adres for an extra website.
If you use DNS from MiAB, maybe, you need a second IP adres if you use the box as primary and secondary.
In my case, I use the DNS from my uplink provider as primary and secondary, so, i just copied the DNS Settings, provided by MiAB to the uplink provider.
There is no real limit in the amount of domains and users on the box, except that the user-interface will be a little “less handy” for a lot of domains.