To find 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.
So:
\[ \text{Number of neutrons} = 39 - 19 = 20 \]
Thus, potassium has 20 neutrons.