Mario Kart 8 currently features a total of 36 different playable Characters, including 16 default characters and 14 Unlockable Characters. and several characters obtainable through DLC.

Mario Kart 8 Default Characters

Mario Kart 8 Unlockable Characters

There are at least 14 unlockable characters in Mario Kart 8. You unlock characters by getting Gold in Grand Prix mode. Mario Kart characters unlock randomly as you get golds. Here's every unlockable Mario Kart 8 character:

Mario Kart 8 DLC Characters

Nintendo has announced new DLC Packs for Mario Kart 8 that contain additional characters, some from outside the Mario games. According to Nintendo, the first DLC pack will be available November 13, 2014, and includes Tanooki Mario, Cat Peach, and Link from The Legend of Zelda. A second DLC pack releasing in May will include Dry Bowser, along with Isabelle and the Villager from Animal Crossing. Amiibo functionality is also coming to Mario Kart 8, as a method for new unlockables.

Evidence of Additional Unlockable Mario Kart Characters

In a publication of the Official Spanish Nintendo Magazine there was a feature on Mario Kart 8 that included images of a bunch of characters from the game, and one of which was Diddy Kong, who is yet to still be officially revealed. You can see Diddy Kong in the bottom right in the middle of all the characters in the image below:

This may suggest there will be additional characters to unlock in the game after it releases.