OK, so posture checks are only done on Windows, OS X and some Linux platforms. So you'll want to guestimate how many of those devices you'll see vs mobile devices.
You have an option of getting enterprise licenses which cover guest, onboard, and onguard so that you don't need to worry about exact counts for each type. Just keep in mind that each feature uses 1 license.
So a device that is both Onboarded and checked for posture will consume 2 enterprise licenses.