>How will "SM Sentry" determine that Machine is not non-corporate when it enrolls for the first time ? By checking first with Azure AD ?
Depends on how you configure it. But if you have it setup for a user to authenticate against Azure AD, then that authentication will need to be completed before the client can finish enrolling.
>System Manager license is 80$/MR for 3 years, does it mean we can enroll as many Windows Device as possible ?
The Systems Manager price is per managed device, not per MR. So if you have 100 Windows machines, you'll need 100 licences.
You could request a trial of Systems Manager to see it working for yourself.
https://meraki.cisco.com/form/trial/