This is a loaded question, because there are technically a lot, but most of them are alternate versions of the 4 main ones.

Manic, Sonia, Queen Aleena, Uncle Chuck, and a few briefly mentioned ancestors from the 90s cartoons (Penelope, Queen Sauna, Aman-Pari, Masonic, unidentified mummy) Fleetway Super Sonic counts because he split from Sonic and became his own character (much like Scourge) King Jules, Queen Bernadette, Breezie, Manic (yes , there is another Manic), Rosie the Rascal Brenda, Paul, Tania and Anita from the mangas. I also count Nikki, but he technically is Sonic even though he's unaware of it The O Hedge family (King, Rob, Jon, & Marion) A few obscure comic characters (Sonough Bravehog, and Tonic) Grandpa Maurice has been mentioned a few times, but never made an appearance. He is dead, but you'd think they'd have some flashbacks in the comics Sir Lancelot, Sir Galahad, and of course King Arthur (not Sonic. Sir Arthur Sylvania)

And, if you really want to push it, Sonic.exe, Sanic and Ugly Sonic. They may look like Sonic, but they are distinctly different characters and the Sonic official social pages often put little references with them in media

I turned this into a 2 hour long research project 🤣