Hello.
1) Unfortunately, such configuration by default is not available in the Booking Calendar plugin.
In the Booking Calendar Business Large or higher version possible to set the capacity for your booking resource (calendar). So if you set capacity 15 to the booking resource, then the specific time slot will become unavailable if number of bookings for specific time slot will reach the 15.
2) In the Booking Calendar Business Large version, you can set capacities for your booking resources either for the entire day or for specific times within a day. This means you can control the number of bookings allowed for each day or for specific time slots. Learn more about this feature https://wpbookingcalendar.com/overview/#capacity
For example, you could specify that your booking resource can accommodate up to 10 bookings per day in a specific calendar. Alternatively, if you use time fields in the booking form, each time slot can accommodate up to 10 bookings per day. Explore this feature in the live demo https://bl.wpbookingcalendar.com/
For detailed instructions on configuring capacity and availability for booking resources and to watch a video about this feature, refer to this article https://wpbookingcalendar.com/faq/capacity-concept/
3) Please note, this forum is only about the support of Booking Calendar Free version. If you will have any questions about the paid version, then contact here https://wpbookingcalendar.com/contact/
Kind Regards.