Unfortunately, if you need to make the several specific number of bookings per the same time slot at the same date in SAME CALENDAR, it is NOT possible in any versions of Booking Calendar.
A) The ability to book for the specific times only possible in the Booking Calendar Business Small / Business Medium versions (and in higher versions for booking resources with capacity =1). Please note if you will make the booking for the specific time slot, this time slot becomes unavailable for the other visitors for that selected date in the current specific calendar.
In lower versions of Booking Calendar it is possible to make bookings only for restricted timeslots.
You can configure the time slots selections on the Booking > Settings > Form page. Please use the “shortcode generator” on the right side of that page for the correct generation of the shortcode.
You can use one”timerange” (timeslots) or “start time” and”end time” selections or”starttime” and “duration”of time selections shortcodes or even start time and end time entering shortcodes. Please read more about the booking form fields configuration here http://wpbookingcalendar.com/help/booking-form-fields/
You can test the bookings for the specific time-slots in action on the live demo of Business Small version (which is configured by default for the bookings of specific time-slots) here: http://bs.wpbookingcalendar.com/ Other live demo you will need to reconfigure.
B) If you need to make the specific number of bookings per day, then you need to check the Booking Calendar Business Large version, where it is possible to make several reservations per specific FULL date(s) by visitors (not time slots). Please read more about it here http://wpbookingcalendar.com/overview/#capacity
At this version is possible to set capacity of the booking resource, so date(s) in calendar will be available until number of reservations is less than capacity of the booking resource.But in this version is possible to make reservation only for FULL date, not a time slots (time slots in this version only record data in the DB, but do not play any logic in reservation). Please read more info about the booking resources and how to configure them here http://wpbookingcalendar.com/help/booking-resource/
We highly recommend to check this article about different configuration of capacity and availability http://wpbookingcalendar.com/faq/capacity-concept/