Capacity planning is an extension of the My Schedule feature. Under Features & Menu > My Schedule check the box labeled Events are Reservable. Once this setting is enabled you will need to set the maximum capacity of your locations and also specify that you want the session using that location to be constrained (see below).
Set the Session to be Constrained
Agenda > Edit Session
From the edit view of your session, toggle the Capacity Constrained checkbox to turn on capacity planning.
In App User Experience
Capacity Planning Spreadsheet Upload
To set a session as Capacity Constrained via a spreadsheet upload use the column header Constrained with a value of yes/no or true/false. Setting the maximum capacity of a location must be done manually through the cms > locations as shown above.