In the settings of an Activity (Inventory > Activities), you can find the Capacity setting. Here you can decide if the activity has a limited or unlimited capacity. See the screenshot below:
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/session.
If you select Unlimited, there will be no restrictions for how many bookings you can receive for this activity on the same day.
Important: If you set a limited capacity and the product is added to a package, make sure to set the Booking Policy as well.