Bing, 20 is not a problem. As kenc indicated, 20 ducks, if allowed, will create a permanent resident, which will then reproduce to great number. The key is the capping limit, whether they be migratory or permanent dwellers.
20 normally dwells in one area, but as other migratory birds spot the existing permament/temporary dwellers, they will congregate en mass. How's the old saying go? "Birds of a feather, stick together."?
