There are two ways to adjust availability:
For a listing and all time slots that are added to it
Adjust the availability from the Availability tab of the listing editor - this is the default way to set availability and it applies to all time slots of that listing. Here you set the total seats available.
For individual time slots only
Adjust availability from the Time Slot detail pop-up screen on the Calendar page - this will override what is on the Availability tab in the listing editor for an individual time slot. Here you will set the remaining seats available.