When you power a line of connected power rails 17 rails get powered (8 either side of the source).
This is the same for activator rails. I consider this a bug and not a feature because why would you ever want that many powering on.

Say i have a line of 16 activator rail linked together. Each one will eject a entity to a block to the side (normally diagonally) and i want the very last activator rail to light up, I would have to add 8 more activator rail and power the last one just to get the 16th one powered or say i want one in the middle powered, how can this be done without powering one 8 blocks away.

Activator rails should only be powered individually and not by linked rails. There is no point within the game where having a line of them powered by one source would make any sense.

I do hope this gets changed, no idea how long it's been in the game for...