The small circulation pump continuously filters water to keep it clean. In addition the pump also provides the necessary water flow to the heater to maintain your set temperature.
If your hot tub does not have a circulation pump or a low speed pump then the spa uses a larger horsepower pump to provide filtration and water flow for heating.
How many pumps should a hot tub have?
Again, this depends on you and what you need the tub to do. Generally hot tubs have one or two main or Jet pumps. Air blowers can help to boost performance of the jets, while circulation pumps have no effect on the performance of the spa or hot tub.