Update: The latest update as of Feb. 2021 has successfully fixed this achievement for some players. I had wins with a bunch of characters, but they weren't tracking since the Dec. 2020 update. After downloading this most recent update and launching the game on Series X, the achievement automatically popped as soon as the main menu loaded. However, the achievement still appears to be buggy, and some players report in the comments that you might need to purchase and win with new characters post update for the achievement to track your wins. Leaving old info crossed out below for the record.
To unlock this achievement, you need to win at least
ONE match with 8 different playable characters.
The game offers you 6 playable characters from the get-go: Anvil, Ronin, Saint, Dallas, Dima, and Trench.
To play with the other 2 characters you need, you will need to either purchase one of the Founder Edition Packs of the game (which unlock you some/all characters) OR buy them with your in-game earned Reputation points.
You earn Rep points after completing a match or by completing daily challenges seen in the main menu screen.
If you're trying to get all other achievements, you will easily earn enough Rep points to buy the 2 characters you need without spending real money (the cheapest characters cost 15,000 points).
As of Dec. 2020 this achievements seems bugged/glitched for SOME people. It works completely FINE for most people, but after checking on different forums and experiencing it myself first hand, ONLY SOME players might not unlock this achievement after meeting the requirements. Lancer was my 7th win (which registered according to the achievement tracker), but victories with Scorch, Phantom and Chaac didn't count on my end, leaving my achievement progress stuck at 87%. Please feel free to leave a comment if you have any more info about this issue