Ducks prefer to be with other ducks, so you should have at least 2 ducks, plus they're so cute hanging out together it's ridiculous. Having chicken buddies are better for the duck generally than being all alone, but still another duck is optimal, even a different breed. Case in point:
I raised a Muscovy duckling with some chicks his same age. He grew so much faster than they did he kind of "took responsibility" for them and I thought everyone was happy. He was too young/small to be with my older Pekins but even when his size caught up they had no interest at all in each other. But when he got big enough and his hormones started kicking in, he began trying to mate some of the larger chickens. He was really rough and I was warned by BYCers that he could kill the girls and it certainly looked like he would. He also became nippy with the chicks he was raised with. He seemed lonely, not having somebody like him to hang out with. I got him a female Muscovy, but he was still more interested in mating the chickens than her. I think his identity is still really confused now. I still have to supervise him when I let them all free-range or he is giving waaaay too much attention to the hens.
My 3 Pekins and my chickens mingle peacefully but each know their respective sides of the pen and coop/duckhouses at bedtime. My big drake is better than the roosters at warning everyone of danger from the sky.
Have fun! And if possible, build for more than what you are planning for now because as everyone here will tell you, the little buggers are addictive!
