Minimum 2, preferably 3. How many maximum would be dependent on your space available.
Ducks are pretty darn cold hardy. As long as they’ve got ample shelter, they should be alright.
Ideally, you’d want something like a kiddy pool for them to swim in, available as much as possible. At least a couple times a week.
Chicken feed is fine, but you’d need to supplement in niacin - they need it to keep their legs strong. Usually people add in nutritional yeast, 1 tbsp per 1 cup of feed, for niacin. Or yes, an all-flock feed would be nice.
Most domestics do not roost, no, but I believe Muscovy and other wild-like breeds do. They won’t necessarily use nesting boxes, either.
They at minimum need a deep water dish while eating, so they do not choke, and to clean out their nares / eyes.
Also, I’d only get female ducks if you’re keeping them with your chickens, as male ducks may attempt to mate the chickens, and could injure / kill them in the process.