I agree that planning is about as important as anything else. How many trees you can plant depends on how large you expect to grow your trees. Here is a chart I created showing # of trees per acre based on a square pattern.
Also, I think it is advisable before you plant an orchard, to plant a few trees each of whatever you are planning to grow and see how they do over a season or two. I started my 3 acres growing trees 15' apart in rows 20' apart. Given the struggles I have had with my fig trees in winter, I've changed my thinking on how to grow the trees and am going to a 13'X13' spacing. I am fortunate in that I do not have a lot of trees which need to be moved to affect this change which weren't otherwise going to be dug up. Also, I am fortunate I realized this early before I had planted all of the trees I wanted to plant and before many of the trees were too big (not an issue for me currently).
Changing the spacing helps me in other ways. The land I have has been stripped of any life by corn farmers before me. I need to bring in large amounts of organic material to amend the soil. With the closer spacing, I will still be growing on less than an acre by the end of 2014. So amending the soil will be easier... just because you have that much land, doesn't mean you'll want to plant on all of the land.
All of the decisions you make, weigh on one another. Planning will save you a lot of work in the long run.