There are 2 debates scheduled.
One debate is 11 select candidates, the second debate is 12 other candidates. None of the candidates will participate in both.
These debates will be "softball" debates, where they will ask bad questions that appeal to the average voter (who doesn't actually care about politics), and based on the response that the average voter gives, some candidates will not be invited to the next debate (the 3rd debate) and will soon after drop out completely.
The 3rd debate will do the same exact thing that the first two did, but this time, each candidate will get a spot to explain themselves and their platform. Now, the average voter realizes....some of these dems have the exact same ideas. These candidates (the unpopular ones, ie: not Bernie, Biden, or Warren) will not get invited to the next debate.
Now we are at the 4th debate, and you have your most popular candidates + the next three or four candidates that are polling "well", meaning barely reaching 1-2% for the state. This will be the start of the real debates. These will be specific topic, meaning ONLY FOCUSED on climate change, terrorism, money, healthcare, whatever the DNC picks - but only ONE topic. This debate will instantly knock out the last two candidates that aren't polling well, meaning the last two guys that got picked beforehand.
Now, its the 5th debate. There are 4 candidates left. The debates from here on out will be more hard hitting and personal questions. Instead of things like, "What are your thoughts on gun ownership?" the questions will be more like, "If you were President, how would you work with Congress in order to prevent possible foreign military action?" Some candidates won't have good answers, others will. The point is for it to make each candidate look as most electable as possible. Some will suck at this, and they'll lose points (look at the 2016 election, specifically Chafee's responses), then its the final two or three candidates.
Final debates, more hard hitting questions, but more like 10 climate change questions, 10 healthcare, etc.