This is applicable to equipment such as hooded hairdryers that are shared between staff and perhaps used for several services.

Within Settings > Equipment, click [New] to add a new piece of equipment.

Enter the name of the equipment, and the quantity of it you hold. Make sure to select all of the services that it is used for.

Services attached to the equipment will be limited to not allow more appointments at any time than there is equipment.

If you try to manually book against this rule, the system will warn you about the double booking:

This appointment overlaps with another appointment, spills over the end of the day or uses equipment that is unavailable. Do you want to book anyway?