I have done this over the years many times and my procedure is very simple.
I go and look, I find a spot I like, I find the land owner and politely ask “May I put x hives there, and give you a jar of honey per hive”? I have been refused, I think thrice, and succeeded dozens of times on the other hand.
What do I look for? Well it depends on which crop I am aiming at, obviously there is no point in looking for heather in arable farm land and so on.
My primary want is good access. I want to be able to drive in as close as possible to minimise the hive humping. That is just basic logistics. I want a water source which in this damp land of ours is not difficult. And I want enough forage for the hives I am planning on setting out. It’s self defeating to place 40 colonies on five acres of OSR.
The answer to how many colonies will this site sustain is to try it. It truly is that simple, and there are times when there are surprises. I had a wonderful site for five years where after the OSR went over another flow began, and I will tell you honestly I never found out what it was, but it came on stream every year and kept the bees busy for a month, and after that they went to the moors.
Manley said and I cannot disagree, it takes five years to truly assess a site, and to discover how many colonies it can keep successfully. For most it is of no importance as most sites will handle a couple of hives and give a decent surplus. The trick is if you are working on a larger scale is to increase the numbers until the return (weather being considered) begins to diminish.
Finding sites is not really difficult if you are willing to put in the legwork and do your research, but that applies to a lot of life really. 🙂