Looks like five more racers are coming to Mario Kart 8 Deluxe at some point.
Mario Kart 8 Deluxe Getting Five More Characters in Future DLC Waves - IGN
Mario Kart 8 Deluxe DLC: All New Tracks and Characters Unveiled - Cheat Code Central
Mario Kart 8 Deluxe Will Receive Five New Characters In Future Waves
Mario Kart 8: Nintendo Announces New DLC courses for Mario Kart 8 Deluxe; Details here - The Economic Times
Everything confirmed for Mario Kart 8 Deluxe - Booster Pass Wave 5
Mario Kart 8 Deluxe update rebalances the game and adds 5 blank character slots
Every Track & New Character In Mario Kart 8's Wave 5 Booster Course Pass
5 Characters Who Could Be Next in Mario Kart 8 Deluxe DLC
Mario Kart 8 Deluxe Wave 5 DLC Includes Brand New Course, Three Returning Characters - Game Informer