In call ducks I use 1 drake to 5 hens......(I keep 4 drakes and only 12 hens) But, I will breed each hen twice a season with different drakes.......once in spring and again in summer. In wood ducks or Mandarins it's 1 to 1. In mallards, I try to keep 2 drakes in with 8 hens. If you have a heavy duck (pekins or roeuns) I'd have 1 drake to no more than 3 hens.
Definitely no less than two hens per drake (really only if you're keeping them separate in trios), but it's better to have 4-6 per drake -everyone will be happier.
If you're looking for fertilized eggs (for selling/breeding, etc) than keep them in trios, or keep the ratio a bit lower (but no less than two ducks to a drake!), especially if you raise a heavier breed.
If you're just looking for eggs for eating you don't need drakes at all.
i agree with trios but right now i am a few shy of that and all of mine are runin together so with i belive 16 i have lest say 7 to 8 males in there .. dang now i wornder lol ill have to count .. but as of right now all is fine and has been but spring when breeding ill have thier own pens ..