Even if it's not wholly random, having multiple debates is the way to go. Let's say there is a hypothetical field of 10+ candidates. In this hypothetical field is Joe Biden, Beto O'Rourke, Kamala Harris, Elizabeth Warren, Cory Booker, Amy Klobuchar, among others, but let's go with them as the heavy hitters. Maybe even throw Bloomberg in there for shits and gigs. It makes 100% sense to separate the heavy hitters between 2 debates rather than try to get the "varsity" candidates on the stage together and relegate anyone else to the "JV" debate. For one thing, it allows Dem candidates, especially the most serious ones, more chances to speak on issues and to actually have the focus be on what they stand for, rather than a circus. Second, it'll give the lesser-known candidates the same airspace and screen time as the heavy hitters and we could see development in those candidates, plus legitimate challenges from more specific-issue candidates to the prominent candidates that otherwise might not happen.
The bottom line is that the Democrats should watch what happened in 2016 and do literally everything opposite to how the GOP allowed their primary to be run.