You can pair up to six Wii Remotes and/or four Wii Pro Controllers with the console (not including the Wii U GamePad). If you pair additional controllers, the oldest controller pairing data is deleted and replaced with the newest data.
What to do:
While on the Wii U Menu, press the SYNC button on the Wii U console to display the controller pairing screen.
You can also pair controllers from the HOME Menu
Press the SYNC Button on the console again until the screen shows the controller type you want to pair.
Press the SYNC Button on the Wii Remote.
The Player LED on the controller will light up when the pairing is complete.
The Player LED indicates the player number of the controller currently in use.