How many neutrons make up one of these potassium atoms

20 approximately.

Explanation:

There are two variables that you could use to calculated: Atomic Number and Atomic Mass.

Atomic number refers to the number of protons that the atom has.

Atomic mass refers to the whole: protons and neutrons.

If we subtract:  Atomic mass - Atomic Number = Number of protons.

For potassium (K):

Atomic number: 19

Atomic mass: 39.09. (we round the number for the subtract).

So, 39 - 19 = 20.

20 PROTONS. Approximately.