for Dave, you know most of what is written below and i know where you are, you can buy leather from wholesalers ... my thought is 3 hides
for other people reading this, it depends on 3 basic things -
1st - the size of the hides. for instance nappa leather / calves leather is about 25sf / hide. regular cow hides can be 45 to 60 sf / hide. most hides average 50 to 55 sf / hide. remember, some parts of the hide include the legs - you pay for every square inch, but these parts aren't very usable ... so don't think of it as a rectangle or something that you can be efficient or rational on how you lay it out. also remember, the edges are often thicker / stiffer.
2nd - it really depends on the consistency of the hide / how many imperfections or holes there are in the hide ... and where they are.
3rd - it definitely depends on what you are planning to do. if you are doing front + rear seats, i would buy 3 hides. if you were doing door panels and other little things, i would buy 5 hides.
keep in mind the price of leather can be anywhere between 10 + 25 bucks / sf ... depending if you are buying it wholesale, retail or from a reseller. so each hide could cost you 500 bucks to 1250 bucks. it also depends on many other things, such as the grade of leather and the finishing of the leather. keep in mind, leather comes in varied thickness, softness, dyeing types, textures ... and finishing. you probably don't want leather finished with wax - the u/v light of sun will do nasty things to it and it will scratch the finish easily ...
i would not look at 'residential' leathers, i would look at aviation or automotive leathers - the finishes will be more u/v stable.