Hi,
If you choose per user, then you need to have enough Windows Server User CALs and RDS User CALs to cover each unique user (human). So if you have 450 unique humans that connect, then 450 of each type of CAL would be correct. Note it doesn't matter how many active users log in from a licensing perspective. Each user that connects, even one time, needs a Windows Server User CAL and RDS User CAL dedicated to them.
The choice between Per User or Per Device usually comes down to which you have more of. If you have many more users (humans) than devices, then usually Per Device makes more sense since it will cost less. Conversely, if you have many more devices than humans, then Per User will cost less.
Per User is the most common, since often users will connect from the Office, or their laptop, or their home, or a kiosk using the web client, etc., so the number of devices is much higher than the number of users.
-TP