OK, so the DNC-criteria have been clarified on what happens, if more than 20 people qualify. This is pretty interesting.
So, currently 18 candidates qualify, which means going over 20 might be possible.
First on the chopping block are the one that ONLY qualify via the donors. That is currently only Marianne Williamson. If it has to be decided between multiple candidates qualifying only via donors, than the one with more unique donors gets to the debates.
If that is not enough to reduce the field to 20, than candidates qualifying only via polls are endangered. Meeting both qualifiers currently seem pretty safe, which explains why Delaney offers to pay $2 to charity from his own money for each unique donor he gets. Anyways, to decide between them, the average betweeen the three highest polls is used (not the most recent). If that is not enough, than the number of polls count where the candidate reached 1%.