how much paint do I need

I agree with having a couple qts. left, for just in case stuff. So 1 1/2 gal base or single stage. The color and brand and color of primer/sealer has a bearing.
Usually takes me 1 gal. single stage to do a complete car (not bottom side though). 3 coats. Type gun and etc makes a difference. I have done some with 3 qts.
I used 2 qts clear on outside last time for 3 coats. I will not sand that. single stage only for topcoat, it took 1 gal.nothing hardly at all left over. ,,, on eng. compartment, jambs, underside of hood, trunk lid, the B body trunk was in spatter paint.
Heck, I see some guys doing 6-7 oats of clear! Really!????