Write the ground-state configuration of neptunium, $\mathrm{Np}$ ($Z=93$).

After radon, the actinide filling begins. The ground-state electronic configuration of neptunium is: $\mathrm{Np}: [\mathrm{Rn}]\,5f^4 6d^1 7s^2$

Count all electrons present in f-subshells.

The question asks for the number of $f$ electrons in the entire atom, not just valence electrons.

• In the core $[\mathrm{Rn}]$, the filled $4f$ subshell is present: $4f^{14}$
• Outside the core, neptunium has: $5f^4$

Therefore, total number of $f$ electrons is: $14 + 4 = 18$

Final answer

$\boxed{18}$

Comparison with stored answer

Stored correct answer = $18$.

My derived answer also is $18$, so they agree.