About 900 plants per acre. About 2 lbs dry hops per mature plant. I just put in an 1/16 of an acre....lot of work, but I will soon be rolling in 50+ pounds of hops a year! Checkout this link:
Crannóg Ales - Downloads Scroll to the hops manual. All the info you need.
I considered doing an acre, then I looked at an acre of hops.....too much for me to deal with.
Good Luck!