How the part fits? A 2" rail end has the inside diameter of 2". It fits over 2" O.D. top rail.
Where is it installed? A 2" rail end is installed at the end of the top rail. You will need a rail end at every point of termination.
How many do you need? For every stretch of chain link fence, from terminal post to terminal post you will need two, 2" rail ends.