Based on the webpage I found, all of the ducklings should imprint on their keepers as Mama duck. However, with ducks mama duck isn't important to a grown duck. That relationship you will have to cultivate separately. If a lone duckling imprints on their keeper without any other ducklings or ducks present then it will think that it is human. This can cause confusion and problems with the full grown duck if it is expected to integrate with other ducks later. If it's going to spend its life as a lone house duck, this might be fine. But keep in mind companionship is very important to ducks so if everyone in your household is off to work or school for 8 hours then a lone duck is likely to be unhappy.