General rule of thumb is 1 drake for 3-9 hens. A drake only needs to mate a hen once every 2 weeks. She retains the sperm that long in her body. A drake may also favor one hen over the rest. Hens are usually more relaxed when they do not constantly get breed and therefore their eggs are more fertile just due to stress reduction. It is ok to have one drake and one hen, if there are no other ducks around. Have at least 3 hens for each drake. I have 8 hens with one drake and fertility runs around 100%. There are other factors, like light, temperature, space, how much they walk, etc.