If you are having to top up the coolant on a frequent basis then I would imagine it is doing on of about three things.
Burning coolant - you would notice white smoke (steam) coming from your exhaust on a constant basis (e.g. not when cold as this would be the cat working on open loop)....this could be the headgasket or cylinder liner cracked
Losing coolant - you would notice a leak on the floor or inside the driver footwell. Also check the engine oil is not mixing with coolant (mayonaisse looking substance).....heater matrix or hose, etc
Coolant pressure cap on the coolant reservoir is defective and the coolant pressure is not being maintained.....this is the first one I would check and also the cheapest.