Step 1: Read the question carefully.
We are asked for the maximum number of electrons present in a single orbital that has $n=4$ and $l=3$. The trick here is to notice that the question says orbital, not subshell.
Step 2: Recall what an orbital is.
An orbital is a region of space defined by a fixed set of the three quantum numbers $n$, $l$ and $m_l$. So one orbital is one specific $(n, l, m_l)$ box, not the whole collection of orbitals in a subshell.
Step 3: Identify the subshell from $l$.
Here $l=3$ corresponds to the $f$ subshell, so this is the $4f$ subshell. The $4f$ subshell as a whole has seven orbitals, but the question asks only about one orbital.
Step 4: Apply Pauli's exclusion principle.
No two electrons in an atom can have all four quantum numbers identical. Within one orbital, $n$, $l$ and $m_l$ are already fixed, so the only quantum number left to differ is the spin $m_s$, which can be $+\tfrac{1}{2}$ or $-\tfrac{1}{2}$.
Step 5: Count the electrons in one orbital.
Because spin gives just two allowed values, a single orbital can hold at most $2$ electrons, one spin up and one spin down. This number does not depend on whether the orbital is $s$, $p$, $d$ or $f$.
Step 6: State the answer.
Therefore, the maximum number of electrons in an orbital with $n=4$, $l=3$ is \[ 2 \] which matches the key.
\[ \boxed{2} \]