My son and family (total of 5) have also moved into a new house with an Aquafine Aqua 2 water softener. They have purchased the "salt" in bulk, however, they are unsure as to the amount of salt required for the system.
Most water softeners have a brine tank that you can dump 50 -100 kilos of salt into. The valve meters the amount used for each regeneration by metering the water that it dissolves into.