You need a 7 pin to 7 pin because all of the pins are in use if you are using a brake controller.
If you aren't using a brake controller, you only need 5 wires if your trailer has surge brakes (fifth wire for the reverse lockout solenoid). You need a 7-> 5 adapter
If your trailer has no brakes, then you can get by with a 7 -> 4 adapter thats included in the hitch kit.