When you are combining different licence types (CALs) the priority of the licences, where they are available is:
- Pre-assigned Named User CAL
- Pre-assigned Document CAL
- New Named User CAL from server (if sufficient licences available)
- New Document CAL from server (if sufficient licences available)
- Available Session CAL
- Available Usage CAL
- If no licence is available the user will not be able to connect
This means that the server will always try to issue a named user licence before using a Usage CAL. The way to manage this so Usage CALs are used is to pre-assign your pool of Named user licence to specific users.
In terms of the criteria for the number of usage CALs then this depends on your usage. If you have the Governance dashboard running in your environment this can help to advise of document usage patterns.
Hope this helps.