Backend license requirements
Note that some competing products require a per-room license/unit. So they may be cheaper to buy, but you need to consider the license cost to get the total ownership costs.
You need one 365 Basic / E1 license (or higher) per 15 units. (If you are using EWS API) If you are using MS Graph API, you will usually only need one O365 license for all your panels.
One user CAL per 15 units.
The service account needs a mailbox and one service account can be used on 15 units.
One suite/workspace license is required for an unlimited number of panels
Microsoft 365 Configuration
Steg 1. Google Calendar Setup