How many gallons does a 18x52 above ground pool hold?

Zero gallons. No gallons can fit into a plane, no matter what units you are using, since gallons are units of volume and must be expressed in three dimensions.

Now, if you were to talk about a pool of 18' (that little ' denotes the measurement unit called _feet_ in standard English) by 52' by perhaps 10' deep, we'd have the ability to calculate an answer. Help us to help you.