How is Horizon View named user counted?

For example, I have 50 users in AD, which 20 users would connect to View as Named User. I buy 20 Horizon View Standard Named User.

Questions:

1. How Horizon counts the named users? The users that had been added entitlement?

2. If a Named User left my company, how to re-assign that lic to another user?

3. If the 21st user login to view, what would be the result? Not able to login or just some kind of warning?

Thanks!!