The 737-400, -800, and -900 have two cabin air conditioning zones plus the cockpit. The rest have one plus the cockpit.
Both packs are on in normal operation because they are slightly more efficient at lower flow rates. It also is safer as no immediate action is necessary to prevent depressurization if a pack fails.
Temperature is regulated separately. The packs cool to the lowest temperature demanded, and unconditioned bleed air, called trim air, is mixed in for the warmer zone.
The dual cabin zone models have three knobs which set the temperature of each zone, from a low to high temperature with a default detent.
The single cabin zone models have a knob for the cabin and one for the cockpit zone. The knob is divided into two ranges, an auto range, where a temperature is controlled, and a manual range, that selects the position of the hot air mix valve in each pack.
The temperature control dials are located in the overhead panel in the cockpit. The pilots operate this, often in response to complaints from the cabin crew relayed over the interphone. (The A320 has an option for cabin crew adjustment of temperature, eliminating the hassle)