I had 6 hens to 2 drakes and it was too much. Bald spots, ducks always being chased etc. Both of the drakes were looking mean. I removed one of them and immediately it all changed. No more bald spots or chasing, everyone happy.
Now I have 4 hens that are going in to their second winter, and from the spring ducklings I have 2 drakes and a hen joining the flock. Hoping that the older hens will keep the young drakes in line.
I don't have experience with the ideal ratio, as I don't keep drakes. But if you do intend on having both genders I definitely recommend not pushing the limits. Personally, I would even have more girls than recommend to be extra careful. My precious duck was damaged by excessive drake attention before I got her over 5 years ago. She still has health problems because of it. Sometimes people can get away with non ideal ratios, but when they don't the results can be fatal for the girl ducks. I don't think it is worth it to find out.