Some painters can dilute wall paint (but not to make it spread over a larger space or for economy), but because sometimes the wall surface requires a less thick application - but perhaps more coats).

Somewhere on the can, in the very, very fine print, should be the amount of square feet that gallon will cover.

coverage depends on the surface...unsealed and textured surfaces will take more paint.. Sometimes paint needs thinned to go on smoothly

Read the can as some paints will say do not thin. Typically I figure around 300 sq ft per gallon, but depends on the paint and the surface type