depends on the guns air pressure and use, if you are using one that needs 50 lbs or more constant pressure that consumes X amount of air you need a compressor capable of keeping that, I had a friend who got away with a small 20 or 30 gallon tank and painted entire cars with that. he would just spray one tank of paint from his gun and by the time he had mixed and filled the next container it had built up enough pressure to spray that.
Just make sure you get a good dryer and some of those disposable water filters for your guns air supply or you will end up with fish eyes in your paint job.
Technology is great when it works,
and one big pain in the ass when it doesn't
Detroit iron rules all the rest are just toys.