During check-in, how do we track room counts, capacity, and then close them when they get full?

In this screen-shot the Barclay family sees Nursery A is full, so they check Jenn into Nursery B.

Create both groups and set the room capacity.

If you need to create the groups from scratch: In the navigation menu go to Groups & Events > Create. On step 1 of Group Creation, make sure in the "Maximum Size/Registration Limit" you set this to whatever the capacity would be for this room. Once you create one group, in that group click the blue Edit > Copy Group, and rename it to your second class. This will copy the settings for the original group you created so that the capacities match.

If you already have the groups created in Churchteams: Go to the blue Edit > Group Information page and double check that you have the correct "Maximum Size/Registration Limit" for each of your groups.
Set them up to mirror each other using Roster Syncing (Edit > Roster Syncing).

This assures that every child enrolled in or removed from Nursery A is also enrolled in or removed from Nursery B.
Enable Room Check-in Count and Close.

Go to Check-In > Views/Settings and click on the name of the Check-In view that you want to enable this for. Expand Section "C: Family Check-in Screen Options"and select option C8: "Display room count compared to capacity and close check-in when maximum size is reached," and click Save.

If two families check their children into the last spot in a room at the same time from different check-in stations, the one who clicks Print last will see this warning:
Text-to-Checkin works in a very similar way.
If Maximum Size for a room is updated on the fly (phone or another check-in station browser), it will take effect right away. This will open up new spots in previously closed rooms.
To monitor and update counts and capacities for all rooms click the report icon in the bottom right of the Check-in Search page.

Enter the check-in password and select the new Summary / Capacity Report

This will give you an overall view of counts and capacity for all rooms.

To adjust the capacity on the fly, click the Edit button in the header, edit any of the rooms, and click the green "Save" once done.

Reassure parents and volunteers that rooms are not being over-crowded by listing alongside each class, its current count and capacity. Then when capacity is reached, close the room from allowing any other kids to check-in.