Air conditioning (AC) services are usually split into two major categories. The first is standard maintenance, and the second is repair.
Standard AC maintenance is needed every 3-4 months in order to ensure the quality of your AC units. Poor maintenance can affect efficiency, air quality, health and temperature control.
Standard AC maintenance is usually given as a fixed price. But beware of hiring a professional based on price alone. While many companies offer exceptionally cheap rates, they may also provide an exceptionally cheap filter wipe-down service. Proper AC maintenance means removing the filter for proper cleaning, cleaning the drain pans, removing any dust or material inside the AC, a refrigerant gas check and refill as required.
AC repair is more complicated and usually requires an inspection, as the cause will not be immediately apparent.
If you are unsure of how many AC Units are in your residence, simply count the number of thermostats you have. Usually each thermostat accounts for one unit.