For over 20 years, every year, a couple of mallard ducks on my pond

For more than 20 years, a couple of mallard ducks came to feed and mate on my pond of 10m2. I see them a few days, then they disappear.

• I do not understand why there is only one couple.
• I do not know if it is always the same couple and their offspring or if they are new couples every year.

It's always a pleasure to see them.