Some back ground information for you
Fire category is based on the overall lenght and maximum fuselage width of an aircraft. It is not based on load (cargo/pax) with the exception of dangerous goods flights. If a 737 lands at any ICAO airport weather freight or passenger the same fire category must be provided.
There are so set minimum staffing levels set in CAP168 for fire personnel for category 4 to 10. It is based on licensee assessment and approved by the CAA. This means that two different airports can have different personnel numbers for the same aircraft type due to different considerations (appliance type, station location, media quantity, location of location authority etc)
"Tempory Depletion of Services" is allowed by the CAA in certain circumstances by the CAA where aircraft can still operate with a reduced level of cover.
Appliance requirements - Cat 1-4 1 appliance, Cat 5-7 2 appliance, Cat 8-9 3 appliance. There is also specific water and foam requirements for each fire category.
From the above information
Most airbus or boeing aircraft are cat 8 or 9. Any new appliance (Cobera, Sides) or even older can be operated by 2 personnel.
so all required appliance (3) can be manned with only 6 personnel although I would think that the CAA would want between 9 and 12.
Any airport will employ extra staff above requirement due to holidays and sickness. These extra staff and off duty personnel could easily make up the numbers required.
Chapter 8 of CAP168 is a great read
Regards
5Q