Potential Democratic turnout is mostly a guess, but it’s likely to be well above the 123,000 who turned out in the 2016 presidential caucus, which Bernie Sanders won against Hillary Clinton. A working figure is between 500,000 and 700,000, similar to the 636,000 Democrats and unaffiliateds who voted in the Democratic primary for governor in 2018.
And, it could be higher due to the competitive nature of the race, the organization of some candidates, especially Bernie Sanders and Elizabeth Warren, and the advertising of Michael Bloomberg.
When you receive your ballot, there will be several candidates on it that dropped out (e.g., Cory Booker, John Delaney), several candidates at one percent (Tulsi Gabbard, Deval Patrick), and a few vanity candidates (Robby Wells, Pete Kechenovich). It’s likely that Michael Bennet will be on the ballot, but out.