Maximum number of electrons in a subshell with $l=3$ and $n=4$ is:

The subshell with $n=4$ and $l=3$ is the $4f$ subshell.The number of orbitals in a subshell is given by $(2l+1)$:For $l=3$, number of orbitals $=2(3)+1=7$.Each orbital can hold 2 electrons.Maximum number of electrons $=2\times (2l+1)=2\times 7=14$.