Call ducks seem to naturally pair off, much like wild mallards. The gal that gave me mine last year has a whole mess of them, and never pays much mind to the ratio of hens to drakes. Hers (and mine) don't abuse the ladies the same way standard ducks do. They may tussle amongst themselves, which is fine, so long as they leave my hens alone!
Personally, I can't stand having more than one standard drake regardless of how many hens I have. They gang rape the hens, and I have even had one of my hens killed by them. These ducks have a huge coop at night and pastured on a creek during the day. Never again. I butcher them as soon as they hit 6 months.