Realistic and a bit conservative numbers considering SFV has 7 until now and did it releasing on a single console, in a rough state, lacking enough (specially single player) content, with a way lower prize pool, etc., which caused it to launch with way lower sales that it could have by solving all these issues.
Street Fighter 6 doesn't only solve these issues, it goes way beyond starting with a ton of content, apparently in a very polished state, also releasing in Xbox Series (not in XBO) which will highly reduce the toxicity from xbots to SF, with a record prize pool in the genre and a way more established eSports/streaming community and userbase, now Capcom released since over half a dozen years a ton of very successful games in both sales and reviews so people will be more confident with the next Capcom game, now it does many things right to introduce new people to Street Fighter and fighting games, etc.
I think it will go beyond a dozen million units sold games.