Keeping too many drakes in a flock will cause too much competition and over-mating will occur. This is a problem since the ducks can be hurt by repeated mating which can cause injury or even death.
A ratio of 1 drake to every 5 ducks is about right for light breeds however you can keep 1 drake with 1 duck, it should not hurt, just keep an eye on the duck to make sure she isn’t losing too many feathers on the back of her neck / head when the drake grabs her to hold on during mating.
It’s when there is more than one drake that over-mating and squabbling occurs.
Having more than one drake in a pen with the ducks will certainly cause squabbles.
If you have too many drakes, it is better to keep a pen of drakes on their own during the breeding season from February through to September to be on the safe side. During the winter months, outside of the breeding season, they are generally easier to keep as a large mixed flock.