Usually, most swan species only breed once a season, Mute, Tundra, Trumpeter, Coscoroba, Whooper, Black-Necked and Bewick (Eurasian sub-species of the North American Tundra) swans only produce cygnets once a year.

Australian Black Swans produce cygnets twice a year.

Now, having said this, if something should immediately happen to the eggs when they are laid, then all swans have the capability of double-clutching, producing another set of eggs to replace the first clutch. However, this is not something that you would want to occur as the female swan loses approximately 30% of her body weight during nesting. Double clutching can cause great stress to the female to the point that she could become seriously ill from such weight and energy loss and even possibly die. The Regal Swan.