In your activity's settings, you will see a drop-down option in which you can choose between a limited or unlimited capacity:
If you select limited, you can specify the maximum number of available spots.
In this example, the available spots were set to 10. This means that Bookinglayer will not allow more than 10 people to book this activity on the same day.
If you select Unlimited, there will be no restrictions for how many bookings you can receive for this activity on the same day.