To create a booking type that only allows one booking at a time, you'll need to set only a single table to the booking type. This will stop other bookings from being made because the single table will already be occupied.
You can achieve this by doing the following:
Click Settings, Sites, and select your site.
Click Booking Types, and select the booking type.
Scroll down to the Areas for this Booking Type section. and click Edit Areas.
Select one single table you would like customers to book for.
Go to the Auto-Confirm Rules header and ensure that the box for Still take enquiries for bookings if no available areas found is unchecked.
Click Save.
Once you're happy with the rest of your setting in your booking type, you'll need to adjust your settings for that single table so it's capacity is able to hold your desired amount of guests:
Click Settings, Sites, select your site.
Click Tables/Areas, and find the table you've assigned to your booking type.
Adjust the settings for Min People, Max (Seated), and Max People to your desired sizes. This is to stop bookings that are too large or too small from being able to book.
If these settings have been applied correctly, your booking should now allow only one booking at a time.