A machine quantity controlling device which controls a quantity of a heat source device to operate in a heat source system including a first heat source device and a second heat source device, the first heat source device being a waste heat recovery type absorption chiller, the second heat source device other than a waste heat recovery type absorption chiller, the machine quantity controlling device including an acquisition unit that obtains a waste heat utilization maximum load which is a maximum load when the first heat source device receives only supply of the waste heat; a determination unit that determines a predetermined load range from the waste heat utilization maximum load to be a first optimal load range as an optimal load range of the first heat source device; and a machine quantity control unit that controls a quantity of the second heat source device to operate so that the sum of a total of minimum values of the optimal load range of the first heat source device to operate and a total of minimum values of a second optimal load range of the second heat source device to operate is smaller than or equal to a load required for the heat source system, and the sum of a total of maximum values of a first optimal load range and a total of maximum values of the second optimal load range is equal to or greater than the load required for the heat source system, the second optimal load range being an optimal load range of the second heat source device to operate.