If you are using quart bottles, buy 5 cases. If you have a winch and do the winch housings too, you'll have a few quarts left over, no winch and you'll have 6-7 bottles left.
Maddawg308 had the quantities right. The only thing he didn't cover was the winch gear case and end housing. From memory, they'll take 4-5 quarts.
Lube goes bad from oxidation and oild oil will contaminate new oil and cause it to break down faster so although expensive the best way to establish a firm baseline for furure maintenance is to break all the fill and drain plugs loose and run the truck for 5-6 miles to warm up the oil. Then drain the existing oil and refill it with (hopefully cheap) oil and take it back out to warm that up. Then drain it again and fill it again.
That will wash all the old oil and more crud out of the cases. Now you can extend the change intervals based on knowing what is in there, whether the truck is stored indoors, the kind of use (or lack of use) it gets etc.
Lance