Outposts are areas of a town that do not have to be connected to the main town. Thus your town can be say, in the mountains, but you can have an outpost in the nether. It's a town claim elsewhere with the perms of your town.
I think as many as you want, but don't quote me on that. It is useful if you have your town area, then a Nether claim, then perhaps a farm completely elsewhere. You shouldn't use too many probably. You can make one by doing /t claim outpost . Then whatever number outpost it is, so if you make one right now, you can do /t outpost 1 to TP to it.
Well it's apart of the town that you are in, so you can make a outpost and the claim is under UnitedEstate. The claims in outpost are the same as the main area of the town. Essentially, think of it as a regular town claim, but just somewhere else. You can do the same things with it. Hope this helps