A: Total users are total editors per site.
1 basic site can be edited by 2 editors
1 pro site can be edited by 5 editors

At the moment, it’s not possible to assign a user to manage or edit only one specific website—all invited users can see and edit all sites under your account.

We’ve noted this request though, and it’s a helpful feature idea for future updates. Let us know if you need anything else!