How the part fits? The rail end is part of the connection of the top rail to the terminal post.
Where is it installed? Rail ends are installed on the end of the top rail. The top rail slides into the rail end, which connects to the end band attached to the terminal post using a nut/bolt (not included).
How many do you need? For every stretch of chain link fence, from terminal post to terminal post, you will need two rail ends.