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 the each time slot.
Note: If you want to change the availability for all time slots moving forward, you must first change the availability and then select the checkbox "Update all availability on all time slots" such as the image abov. Test this by clicking on "Preview" in the top right corner of the listing editor to see the booking page and what the availability changes to.
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.