number of different items, consider similar items once

Hello there,

I am looking for a formula/function which can give me quantity of different items.

something which gives me 3 (David, Sarah, Mark) if I apply it for Column 1, and gives me 2 (24, 43) if I apply it for Colum 2, on the below sheet:

Note: I don't want to type David, or Sarah, etc, in the formulas because there may be hundreds of those name and I don't know those name either. I just want the quantity of individuals.