In some cases one is fine- but it really can depend on your drake. I have pairs and trios separated out for breeding as well as a bigger flock all together- it all seems to work well here. It can also depend on what you are keeping the ducks for. If you want to ensure fertility in the eggs being laid by the females 3- 4 is a good number. People keep different numbers for different reasons- and it can depend on the number on your entire flock as well. If you just have the single pair of ducks- then the drake has no competition for mating and is more likely to be less demanding. In larger flocks where there are multiple drakes they will compete for dominance and it is in these situations where it is more likely for a duck to suffer the consequences of overbreeding.