Catmandoo has a point, think of this as well, a small pond with a problem costs a lot less to fix than a big pond with a problem (think invasive plants or fishery unbalance). To have all ponds go "bad" at once would be unlikely if diversified.
If I had reasonable topology and did not live on a hill I would definitely have more than one. A quality swimming pond, a good fishing pond or two, and another shallow one dedicated to amphibians, small fish, and wildlife.
