The rule of thumb when buying a heater is 5 watts for each gallon the aquarium holds. Typically, one heater is sufficient for an aquarium. However, for better heat distribution in larger aquariums, especially tanks more than 36 inches in length, it's best to have two heaters.
The heaters should be placed at opposite ends of the tank and each should be about half the total required wattage. For example, if you have a 55-gallon aquarium, you will have the best heat distribution is you put a 150-watt heater at each end of your aquarium.