Even if you aren't breeding them, they're going to breed. I'd recommend the 3-4 rule whether or not you plan on raising offspring if you don't have the ability to separate the drakes if they are getting too frisky.
Though I have had my flock at a 1:1 or 1:2 ratio, I find 1:3 works just a bit better, especially if you have multiple drakes.
Drakes tend to be hormonaly driven in breeding season and can injure a duck by mating with her repeatedly. Some folks separate their drakes if they seem to be overly aggressive about mating, others don't have that ability (this is where I'd recommend the 3-4 rule.)
If you want 2 ducks go for a pair, or 2 hens.
3: 1m, 2f.
4: 1m, 3f.
5: 1m, 4f is my recommendation, but 2m, 3f can work in some flocks, but you may need to separate the drakes for a bit (possibly a few months.)