How many is reasonable to have as a first response,
ICAO Annex 14 has the rules pertaining to how its worked out.
The level of protection provided at an aerodrome for rescue and fire fighting shall be appropriate to the aerodrome category determined using the principles in 9.2.5 and 9.2.6, except that, where the number of movements of the aeroplanes in the highest category normally using the aerodrome is less than 700 in the busiest consecutive three months, the level of protection provided shall be not less than one category below the determined category.
There are rules on the amount of extinguishing agents carried and minimum deployment time.
Typically 4 Fire crew on station with 2 per tender gives an availability of 2 tenders. 6 is 3 tenders.
Depending on the type and size this may be sufficient to meet the required category for the typical user.
If an aircraft beyond the typical is expected then it is possible to bring in extra fire cover prior to the scheduled arrival.
This cost's extra.
Delta paid the airport to have extra crews on duty when they were flying some 747s
Many airports have arrangements where they will accept a higher category with prior notice.