To find the number of neutrons in potassium, you use the following formula:
\[ \text{Number of Neutrons} = \text{Atomic Mass} - \text{Atomic Number} \]
For potassium, the atomic mass is approximately 39 and the atomic number is 19.
So:
\[ \text{Number of Neutrons} = 39 - 19 = 20 \]
Therefore, potassium has 20 neutrons.