You cannot exceed 8 players on Wii Fit / Wii Fit Plus. The software supports exactly 8 Mii profiles per save file, matching the Wii system's Mii Plaza limit.
The Wii console stores max 100 Miis in Mii Plaza, but Wii Fit only reads the first 8 registered to its save slot. You cannot create a 9th profile in-game.
For families >8 people: rotate profiles or use a shared "Guest" Mii (progress not saved).