How is the monthly number of user sessions calculated?

You can think of it being the total number of connections established to Realtime during a given month. It's reset to zero in the first day of each month at 00:00 UTC.

However when you use the Realtime Javascript library there's a slight nuance: all connections from the same browser during a period of 30 minutes will count as a single session.

The sessions metric is really only relevant for the Free plan as all other plans have unlimited sessions.