I had only 1 duck hatch that was on it's own, and then 2 others hatched 2 weeks later from a different batch. They went into different brooders initially and they were still able to see each other, which was important socially for the solo one.
Once the first one was too big to spend the day in a brooder, and didn't need the heat lamp during the day, it moved to a puppy play pen (about 2m x 2m) during the day and back into the brooder at night with the lamp (it's winter here). Once the other two started to get too big as well, they joined the original in the play pen. There was a bit of pecking initially by the oldest, but that has mostly settled and they all get along now. They spend days in the pen and nights in their respective brooders. All 3 wouldn't fit in one brooder at their current sizes.
When they move outside they will be with 2 adult muscovy ladies (who hatched the first one). They seem curious about the young ones so fingers crossed they all get along then too!