To determine the number of neutrons in an atom, you can use the formula:
\[ \text{Number of Neutrons} = \text{Atomic Mass} - \text{Atomic Number} \]
For potassium (K), the atomic number is 19, and the atomic mass is approximately 39.
Calculating the number of neutrons:
\[ \text{Number of Neutrons} = 39 - 19 = 20 \]
So, potassium has 20 neutrons.