How many AC compressors and evaporators does this vehicle have?

Your vehicle's A/C system consists of one compressor, condenser, condenser fan motor and a series of sensors and relay circuits. If your vehicle is equipped with an auxiliary cooling capability typically in the rear of the vehicle, then you have two evaporators and two expansion valves. Otherwise your vehicle has one of each. Depending on how your vehicle is equipped and what the symptoms are and diagnosis, the investment to make the repair could be approach what you were told. When a repair exceeds $1,000 to $1,500, I would suggest getting a second estimate and use RepairPal as a source to gage each accordingly. I would also have an A/C service performed prior to the repair to help pinpoint the challenge; this consists of evacuating the refrigerant, performing a leak test and recharging the system with refrigerant and dye combination. This way the leak(s) can be detected with a special light. The cost is approximately $100 to $150, I hope this helps.

I think your mechanic is correct, sort of. As stated previously, some systems include a rear evaporator core and expansion valve. My '02 Sedona has rear A/C and heat. It's on the passenger side in the back. If you look underneath beside the spare tire, behind the passenger side rear tire, you should see bolted-on plate on the underside of the floor with 4 pipes going into it - 2 steel coolant lines (probably rusty) for the heater, and 2 silver lines with insulation for the A/C.