This command finds the cardinality of the set that is associated with the given
key. Cardinality is the number of elements in a set.
keydoes not exist, 0 is returned.
keyis associated with a non-set value, an error is raised.
Returns the cardinality of the set.
$ SADD yuga_world "America"
$ SADD yuga_world "Asia"
$ SCARD yuga_world