max clients on IAP is configurable up to 255, but is 64 by default. Of course, I would caution you about pinning that many clients on an IAP(s). We always need to ensure proper capacity and client experience. In current code, we QA up to 64 IAP's in a cluster and 1048 clients.
At the moment, there is not different tiered access for login to the IAP but its something that has been submitted on the idea's portal.