That's really not the use case for Prime. There are other Cisco solutions that can help you count / inventory equipment.
If you want to budget, you really need accurate switch counts and switch types.
Alternatively, you could do token based licenses, but you are going to have to purchased at least 20% more tokens than what you need.
You'll need to go switch to switch anyway. If you are not on the right software version, Prime may not be able to manage the equipment.