CountDistinct

CountDistinct[list] gives the number of distinct elements that appear in list.

CountDistinct[list, test] applies test to pairs of elements to determine whether they should be considered equivalent.

Examples

Count distinct elements:

CountDistinct[{a, b, a, c, a, b}]
(* 3 *)

With duplicates:

CountDistinct[{1, 1, 1, 2, 2, 3}]
(* 3 *)

Please visit the official Wolfram Language Reference for more details.