Hi
@FenrisMau
I don't have that many ducks, but I'm guessing you will need at least 200-250 gallons, give or take a few.
I use kiddie pools for my 6 ducks and I know that one duck fits comfortably in about 25-30 gallons of water. A goose preferred about 50 gallons.
The pond should be at least two feet deep, preferably three, in the deepest part and it appears that ducks especially like contoured edges. Each duck should have about 4 sq. ft of surface area; a goose should have about 6 sq. of surface area.
This measurements are based off my observations of my own waterfowl, so it may be somewhat different for yours

. Hope this helps!