As far as we know, the number of available settlements at domain selection will be "random", from 0/1 that point is not totally clear, to 75, that point is not clear either.
0 comes from an answer from Caspian in a Q&A where he says that some county may have only one settlement or even be empty.
75 comes from DJ#18 where Caspian said that Counties would have a max population of 750, and a hamlet, the smallest settlement, has a population of 10.
it is not clear if a minimum size will exist for a settlement to be available at domain selection ( can people select a hamlet or will they have to pick at least a village or even a town ?)
We know that since there are more counties than what was described in DJ#18 they are going to be smaller than the average described in the DJ.
From there it is all guesses and assumption, but the most general idea is that populated counties will have from 100 to 200 inhabitants, with one to 3 town level settlement, maybe for a few a city instead, and 1 to 3 villages, the less town, the more villages, so yes 5 settlements on AVERAGE.
But remember those are guesses and assumptions, the real answer is, no one knows yet how many settlements will exist in a county because those are going to be generated proceduraly during the world generation process, it will be a case by case situation as each county will be different.
IF more than 2 or 3 Aristocrates have pledged to a count they must be ready to move.