Cars shouldnt 'consume' coolant, apart from maybe a very small amount over a very long period -which shouldnt be apparent if the coolant is changed at the recommended periods. If it loses a significant amount there are two possibilities
1) There is a leak somewhere. Could be radiator, faulty pressure cap, damaged rubber pipes, leaking 'water' pump, head gasket, etc. etc The leak should be fixed, not simply topped up.
2) As others have found the level may drop a short time after filling the system then settle down. Although the header bottle may have a max and minimum line it doesnt mean it must always be on the max line ,or that you need to constantly top up to maximum. The cooling system has its own 'happy medium' where it has just the right quantity of coolant to fill all the pipes and run at an ideal pressure. This level should be somewhere between max and min. If you fill to max it might actually be too much coolant, causing excess pressure in the system. The system will vent this excess coolant ,through its pressure cap until it falls to the ideal level and then remain constant. Its easy to mistake this drop as a leak ,and keep topping it up to max, only for it to be vented out again in a never ending cycle.
The same can also happen when filling the system if you get air locks in the pipes. After a few miles the air will bubble out to be replaced by a similar volume of coolant, so the level drops. (on some cars air locks can be difficult to eliminate and cause cooling problems. )
Of course you should check the coolant level from time to time to ensure its not leaking more than normal ,ie dropping below the minimum line. But if the level remains constant at a level somewhere between max and min ,its fine.
Probably going beyond the question here, but if you do top up use the correct type of coolant for the car. Typically there are two types , OAT (usually red) and others that are usually blue/green. Dont mix them . It also comes in two basic formats , 100% antifreeze which will need diluting or as 'coolant' which is typically ready diluted to 50%.
The car takes coolant. Dont top up the system with water , except in an emergency, in which case you will be need to drain and replace the coolant anyway, as part of the repairs.
Couple of other tips,which may help some. Dont try and plug coolant leaks with 'rad weld' type products, unless its literally a case of keeping an end of life car out of the scrap yard for a few more months - and you are prepared to take the consequences and possible dangers of a seized engine miles from anywhere or on the motorway.
Some products still use quite crude ingredients. (Back in the old days Mustard powder was often used, it dried solid as it leaked out - its works quite well
) These products wont work for some leaks, or might work too well, clogging the radiator etc.
The source of a slow leak may not be obvious as wetness or steam. Steam is actually invisible (its the slightly cooler water vapour you can see) and the coolant may dry as soon as it emerges. A slow leak may reveal itself as whitish staining.