Mario Kart 8 Deluxe update adds 5 more extra character slots beyond Birdo

It would appear that Nintendo will include six new characters into Mario Kart 8 Deluxe

Mario Kart 8 Deluxe Will Receive Five New Characters In Future Waves

Five more characters coming to Mario Kart 8 Deluxe expansion pass

The Retrogaming Times - Thirty-Ninth Issue - July 2022

Mario Party 7 (GameCube, 2005) for sale online

amiibo (Toys) - TV Tropes

Mario Kart 8 Deluxe's Character Menu Now Has Five Extra Spaces For Future Updates

Mario Kart 8 Deluxe's new character select screen hints at more Booster Pass racers - Gamepur

Bowser, MarioWiki

Mario Kart 8 DLC hints at more returning characters after Birdo reveal