For that airplane, you probably need one for the aileron to receiver so you can connect and disconnect between flying sessions as you remove the wing for storage and maintenance. If you are using one aileron servo, then you need only one. If you are using two servos, then you need a Y-Harness.

It is important -- to me at least -- that I can hook and unhook with minimum efforet and be reasonably assured that there will be no bouncing wiring within the fuse. servo area during maneuvers.

Bring the one or two female plugs to somewhere where the servo male plugs reach with some excess for working fingers. Secure the female plugs to some part of the side of the fuse, or even a cross member as you may need. I use a 1/16" scrap to CA the females to, the fuse or whatever I select. That provides some jiggle room. A small of scrap cut 1/16 x 1/8 +/- along the sides helps to keep you from breaking it loose when you plug in the male from the servo. Everything is then secured.
EXTRA: Try to place the female so that when the wing is fitted down the servo wires of the male are slightly bent over thus preventing their slipping back out until you pull them out.
Always make a BIG spot of red or black ( preferred since the black wire is always external) so that you can always just go black to black and not have to eyeball so much which way the plug-in goes. Makes less effort! My thing!

If you have servos in the wing then you may need a long extension in the wing. Use heat-shrink tubing and shrink with your monokote gun or a hair-drier. Only need enough to secure those square corners. Or tie them together using dental floss between the wires. Works.
I have models many years old and have not experienced Rodney's problem. Now, after saying that, will I be able to say same tomorrow evening? OH Lordy!! []