If you can handle more bookings at the same time, you can increase the occupancy number thus allowing more customers to book a service on the same date for the same time.
Navigate to Settings - Online Booking - General Settings
Select Occupancy
The default number of bookings is set to 1
You can select directly 1, 2 or 3 bookings. If you can handle more, click on Customer number of bookings and select a higher number from the selection and press Done.
Go Back and click on Save