I wouldn't suggest planting them directly in the ground. Personally, unless it is on YOUR property which has been your property for a loooonng time, and you live uphill from everything else, i just wouldn't plant them in the ground. Unless you have the capability of running maad soil tests and all.
My thing is, im an organic smoker and i like to know EXACTLY what im smoking. The thought of my potential smoke, which i will be inhaling into my lungs, growing in a nutrient sink from all the wrong places is bad idea. It's very difficult to find soil which will support your needs. What you could do is dig holes in the ground and fill them with your choice of soil mix. I would just use 5 gallon pots, at least, and let em rip...if you gotta lot of deer in the area or bugs and what not, you have to take all of this into consideration.
Outdoor grows can be very simple, but once you start investing all sorts of money, there are a lot of tedious steps that you are gonna have to take to ensure that your bud comes out the way you want.
But if your takin some bag seeds and throwin em outside then good luck!
Also if you were trying to ask how many Auto plants it would take to achieve a pound, The average rate is about 1 ounce per plant, I would say 20 to be safe.
Happy growing though!
-moose