Capacity planning is an extension of the Agenda feature. Under Features & Menu > Agenda, check the box labeled Allow Session Registration. If you are using Certain Platform for session capacity management you will need to select "Platform Managed". If session capacity is going to be manged solely in Touchpoint, select "Touchpoint Managed".
Set the Session to be Constrained
Agenda > Edit Session
From the edit view of your session, toggle the Registration Required and Capacity Constrained checkboxes to turn on capacity planning for that session. If capacity is being managed directly in the Touchpoint dashboard you will set the capacity for the session in the "Capacity" entry field. If capacity is being managed by Certain Platform you can ignore the field.