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.