The Booking And Reservations extension has its own Quantity option (in Edit Product-> Booking tab). What is it for?
This option defines how many reservations can be taken for the same item for a given time slot.
E.g., you have 2 identical bikes for rent, on daily basis. It means that for a single day, you can only take 2 reservations for the bikes.
The Quantity option is aimed to set this exact restriction - if a bookable item has limited capacity, this option defines the maximum number of reservations for a single period.
This option works independently from the options in the native Inventory tab, and it does not limit the general reservations quantity - e.g., if both bikes have been hired for Monday already, other store visitors will still be able to reserve them for Tuesday, Wednesday, etc.