The new "commentaries at a time" feature shows zero after adding a Commentaries section to my custom guides (PG & EG), but 10 are still displayed.
Confirmed. It's almost like the 1 was simply missed! Entering a new value does work as expected...
It sounds like the edit box is too narrow to show the full number. We'll take a look.
This should be fixed in 6.10 RC1.