E.g. if you need 10 different users to have their own logins, you need to get one 10-user license.

Any disabled user account and account that cannot login does not count against total number of user licenses.
E.g. if you have 6 enabled accounts, 3 disabled ones and 2 which cannot login, you need to have a license only for 6 users.

You can "mix and match" licenses. For example, if you need 16 users, you should get six 1-user licenses and one 10-user license.

You can purchase additional user licenses any time you want.