Choose which days each service runs, close dates or departures and cap the number of people on a given day.
Availability is set per service. That way you can, for example, run "From Cancún" every day and "From Tulum" only on weekends.
On the tour page, Services tab, click Availability on the service card.

Under Operating days, check the weekdays the service runs and click Save days. Days you don't check show on the calendar as not operating, and customers can't pick them.
On the calendar, click the day you want to change:
Click Save.

In the same window you can enter the capacity: the maximum number of people for that day or that departure. Leave it empty for no limit.
If you don't enter a capacity, the service has no limit on people. Set the capacity on dates when your space is limited, for example by the size of the boat or the vehicle.
If you stop selling the tour for a while, instead of closing day by day change its Status to Inactive in the Information tab. The widget will stop showing it until you activate it again.