Book Writer's answer is good. A couple of additions:
You do not have to buy a full 10 yards - that's typically the most you can get in one truck, but the minimum varies from company to company. I've ordered trucks with 4 yards in them. Too low and they'll charge you extra for bringing it to you.
You can figure the volume yourself pretty easily. A cubic yard is 3 feet by 3 feet by 3 feet. 3*3*3 = 27 cubic feet. A 10 foot by 50 foot by 6 inch driveway is 10*50*1/2 = 250 cubic feet divided by 27 cf per yard is 9.26 cubic yards but you really want to order some extra.
Finally, ask how long you get to unload - most give you like 5 minutes per yard. If you can't empty that 4 yard load in the 20 minutes they start charging by the minute and it's very expensive. If you can easily get the chute to where you're pouring you might do O.K. if you have to haul it in wheelbarrows you better get a bunch of friends over, if it's too far or over rough ground you'll be hiring a concrete pump. If you're hiring a concrete pump does the operator place for you or just run the controls - you do not want to be on the business end of a concrete pump with no experience.
Oh, now I see - 4" thick. So, 81 square feet per cubic yard, but order 10% extra for bad measuring and slop.