If they schedule it over the weekend, I think they'd be pushing close to a million people downtown. Bucks were reportedly half a million, and that was still during the pandemic with a much more localized fanbase. Brewers will draw not just the city of Milwaukee and the burbs, but the entire state and even out of state. I'm not especially well versed on the history of crowd sizes, but I would venture to guess it would be the largest single gathering in the history of the state given the two recent Packer parades have been in Green Bay in the middle of winter.
Assuming they clinch it tonight or tomorrow, I personally vote for a Saturday parade down WI Avenue capped by a public ceremony at Summerfest as that holds more people than Am Fam.