An 80lb bag of concrete mix only does about .6 cubic feet (according to an online concrete calculator). To get one cubic yard of concrete, it would require 45 bags. Figure 4 foot deep, 1-2ft wide at bottom, and 3-4ft wide at the top, and you're at a cubic yard pretty quickly.
I believe my pole for my 10ft dish is sunk a little over 4 feet, but less than a foot diameter at the bottom, and maybe 3 feet around at the top. I have no idea how much concrete is actually on that pole, but I wouldn't be suprised at it being a cubic yard. Of course, I've also put a 7.5 foot mesh dish on a wooden fence post with no concrete (just tamped down dirt), and it worked (for analog c-band) for the year or so that I used it there.