Hello.
Such capacity possible to set per calendar (it is means per days or time-slots (if you use the time fields)). And it’s means that such reduce of availability such as 20 – 3 – 5 = 12 possible via each day or via each time slots. 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. This flexibility allows you to manage bookings effectively, whether they are for full days or specific times.
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/
Kind Regards.