How many material elements are ideal for human character

how many material element are ideal in your opinion for a clothed human character ? for me right now its 13 but i want less , keep in mind my game is an open world game with a lot of objects

If your character is modular, ie head, torso, hands, feet then one material per skeletal mesh would be ideal.
Otherwise, the less the better. Typically there is one for the body and another for the head part of the mesh.
If you have more equipment to show on top of the main body, then this adds up to how many equipment pieces you have in total; just remember to keep the material count of each mesh to the minimum of 1.