Ambulances are not and should not be covered by the NEC. Using the NEC as a design manual for ambulances is neither prudent nor safe.
Chapter 3 wiring should not be used. Automotive and marine wiring is made to be flexible, sustain vibration and has insulation designed to be used without a raceway. Solid conductor wiring should NEVER be used. Certain insulation types should never be used.
Obviously, an ambulance won't have an earth grounding electrode.
Automotive wire diameter seems too small by NEC standards. Usually at least by 2 AWG sizes.
What needs to be considered is the load that will be used, not the number of receptacles. If the load will be no more than 10 amps on a 20 amp inverter, what harm would it be to have 50 receptacles?
If the customer has the need to plug 25 devices in, they will do it, regardless the number of installed receptacles, so you may as well give them what they want. If you think that more receptacles will equate to more load, then both the shore power source and inverter will need to be upsized, and limiting receptacles won't affect that. One could plug two 1500 watt space heaters into a single receptacle and overload a 20 amp circuit.