You're correct in that you would get one Tableau Server license that comes with the Viewer licenses pre-loaded. There's a Licensing screen that can be checked which should reflect the current count so as you assign licenses out to users you'd be able to see how many are remaining.
I have another question. Tableau used to have core-based and user-based licensing before the subscription model. As i understand the did away both of these and everything comes under one subscription based model. Is this correct?
For that question it's probably not a bad idea to confirm with your account manager. That is, for existing "core" and "user base" licenses folks can continue with those if that's what they already had and can upgrade to newer versions of the software. I've seen some confusion where folks think they can only go to the 2018.1.x and above series *unless* they're on the newer role based model and that's not really the case. But I'm not sure if for example they're selling the other two types to *new customers*. This thread also has some information along those lines: