You're asking a lot of questions that really depend on what your parameters are.
How much space do you have?
Water access? Time? Shelter? Predators?
Local laws? Maybe you should wait and really think about this.
Having pets/livestock means you are responsible for living things that depend on you for almost everything. you have to let them out every morning and lock them up at night so they aren't eaten by things in the night. there's Feed bills and maintenance. poultry is crazy messy!
These are the ducks from your other thread? The pekin drake and mallard hen? I would say get young adults so they can be sexed, and all hens, so he can spread the love around a bit. Or, if you don’t have your heart set on keeping your pekin fella but want to keep a drake, rehome him and get a Cayuga drake and some more ladies to keep him company. I have horrible luck when hatching sometimes, out of 3 hatches I wound up with 80% drakes!