How much paint do I need to repaint the outside of a 10' X 50' mobile home with a 12' X 12' addition?

You might get by with 6 gal. but I'm gonna say 7 just to make sure AND this is ';per coat';. A lot is going to depend on the the quality of the paint you purchase, the condition and texture of the surface that you are painting and if one coat will cover it. If you purchase a decent quality paint and the surface is in fair condition and you are not doing a drastic color change then you should get by with one coat. Good Luck