The ratio, in general, is one drake for two hens. That being said, I do know someone who has two drakes and three hens, however they haven't done any real serious, heavy mating(they're still pretty young). She's still going to separate them during heavy mating seasons either way, just in case. I have one drake and four hens.
You can have two or more drakes easily living together without any hens.
As far as food, call whatever supplier you plan on purchasing from and asking about aflatoxin in their corn. Aflatoxin is very harmful to ducks and is also a carcinogen for people. Read more in this thread:
https://www.backyardchickens.com/t/711263/u-s-drought-fallout-might-it-affect-all-of-our-ducks
There's reason to believe Dumor doesn't contain enough Niacin for ducklings. Read more in this thread:
https://www.backyardchickens.com/t/826913/dumor-chick-duck-start-grower#post_12059752
Aaaand, this wouldn't be complete without a good list of treats you can give to your ducklings! Read here:
https://www.backyardchickens.com/t/242460/the-ultimate-list-of-duck-treats-and-supplements
Ducks are a ton of fun, but they are work, and you definitely have to make sure they have lots of water(which yours clearly will). I've had a lot of fun watching my ducklings mature into beautiful juveniles, and I'll have even more fun watching them mature into adults. They're soooo rewarding! And even though they've been work, they really haven't been too difficult to care for.
I don't own Calls, but I know that Call owners do love them. I, personally, decided against them due to their noisiness, but I live in a suburb, and we would have definitely recieved complaints.