Well most likely she won't sit on 1 she'll build a clutch first which could be 4 to 20 eggs[you take the ones you don't want her to sit out once she begins to brood] think about it do you know 20 people who want ducklings or can you keep that many[ a little exaggeration] but if you can keep them what happens to them? So start marking the eggs starting with 1 that way if you want to limit how many she sits take out the oldest ones first. then on day 7 candle and that way you can tell which are growing and toss the ones that aren't, she won't begin to incubate the eggs til she has the amount she is happy with. Once she begins to brood make sure she is in a secure place so she won't be killed and her eggs eaten by a predator, then you wait 28 days. make sure she has water for bathing and good feed and she'll do the rest.