What is the cardinal number of a set?

The cardinal number of a set is the number of elements in the set.

Example: the cardinal number of the set {6, prune, 675, biscuit, London} is 5, since the set contains five elements.

If a set contains repeated elements, they should only be counted once.

Example: the cardinal number of the set {6, 7, 3, 4, 4, 7} is 4 (not 6) since the fours and sevens are only counted once.