You would be surprised at how many can fit underneath them! Here's an example using a chicken and some kittens:Maybe trying to keep all those ducklings in line will tire Grandma out and she'll allow the other Mama to help out. I wonder though after learning about communal nests if they both go back to the nest at night and sit together with the ducklings. Surely one duck can't keep 25 ducklings warm and protected all night by herself.