Perez did indeed say only 20 candidates can make it if they meet the rules. For two groups of ten.
Democratic candidates announced or to be announced this week broken into groups of five for visual purposes:
1: Biden, Booker, Buttigieg, Castro, Delaney
2: Gabbard, Gillibrand, Gravel, Harris, Hickenlooper
3: Inslee, Klobuchar, Messam, Moulton, O'Rourke
4: Ryan, Sanders, Swalwell, Warren, Williamson
5: Yang
"Pending" candidates: Michael Bennett
Still "considering" but unlikely: Bill de Blasio, Steve Bullock
I assume Gravel will not make it, which means they will be hard pressed to keep out both Yang and Williamson unless they get all three of those pending/considering in, if only for the first debates to keep out the not serious* candidates. I assumed earlier and still do that the DNC would prefer having Williamson to Yang if they got to pick.
*source: former Senators John Edwards and Hillary Clinton