Depends. Wrap one OG without stretching it real tight and it will make your 1/2 approximately a 5/8. Stretch it real tight and it will be between 1/2 and 5/8.
Most overgrips are about .5mm, so they'll add 1mm to the grip size. 1mm=.03937inches, which is about 1/24th, so if you wrap them tight with minimal overlap you'd need 3 of them to build up your grip 1 size?