How many children did Leda have with Zeus?

Zeus made himself into a swan and then made love to her. He was in the guise of a swan, because he didn't want his wife and the goddess of marriage, Hera to catch him.

As to children, one myth and the one most often told is that she had twins, Apollo and Artemis.

Another version says that Leda had two children with Zeus and two children with her husband Tyndarus, or that Castor was mortal and his twin Polydeuces was immortal. Her daughters Clytemnestra and Helen (yes Helen of Troy) were mortal