I usually figure about a quart (32 oz) to a quart and a half mixed for a coat on the outside of a small to medium size car.
But like vince said too many variables to say for sure. Depends on how you paint, viscosity of material, efficiency of your gun (conventional blowing a lot of material in the air or a good hvlp) ect.
I am opposite. I tend to mix on the short side of what I think I'll need, specially with catalyzed material, figuring I can mix up a little more quickly if I need to. That way if something catasrophic happens in the middle of spraying (which has happened to me before) I am not sitting with a bunch of material mixed up. Then after your first coat, you can look at what you used and know better how much to mix for another coat.
You could try baddbobs suggestion too, if you have no idea. Fill your gun with water and spray the vehicle like you were painting it, to get a ballpark of how much you need. You will have to make sure its okay to spray the vehicle as well, ie if baremetal or plastic filler spots maybe not a good idea, or would need to dry and blow off soon with a blow gun. Plastic filler can soak up water and may not be the best thing if applied over baremetal. Also if fresh paint or primer, that its cured enough to be hit by water and won't water spot. But tryed it once when I was running low on clear and had a few panels to clear. I sprayed a coat of water on the panels and looked at what I used. Then times by 3 for 3 coats. Figured I'd be close but should have enough clear to finish the job. I was shocked at how close it came to the amount of clear I ended up using in my particular instance.
As far as how much material to do a whole car, thats really hard to say, base in particular. You can have a good base and be shooting a color that happens to cover well, an efficient gun, and only end up using 2 quarts to do a whole car (1 to 1 mix) jambs and all, or on the other hand something like omni and the color covers crappy, and burn through almost a gallon (2 gallons mixed).