Looks like five more racers are coming to Mario Kart 8 Deluxe at some point.
Guide: How To Unlock Everything In Mario Kart 8 Deluxe – NintendoSoup
Mario Kart 8 Deluxe's Wave 4 DLC adds Birdo, but also 5 blank character slots - Vooks
Mario Kart
Mario Kart 8 Deluxe is getting 48 remastered courses over the next two years as paid DLC - The Verge
Mario Kart 8 Deluxe Booster Course Pass Wave 5 Thoughts - KeenGamer
Mario Kart 8 Wave 4 update leaves 5 more spots for characters : r/mariokart, smash karts mod menu
Mario Kart 8 Deluxe Getting Five More Characters in Future DLC Waves - IGN
Mario Kart 8 Deluxe - DLC Courses, Characters, and Datamines
Unlockables - Mario Kart 8 Guide - IGN