The #s# subshell has #2# electrons.
The #p# subshell can have #6#.
The #d# subshell can have #10#.
The #f# subshell can have #14#.
This is partially due to the orbitals becoming more complex as they go down so can have more possible orientations in space. For instance, an #s# subshell is just a sphere so can only have #1# orientation in space so can have #1# orbital with #2# electrons, while #p# subshells are figures of eight so can be rotated 3 different ways along #x,y,z# planes, so ca have #3# orbitals.
#d# subshells are yet more complex and have #5# orbitals with #10# electrons, while #f# can have #7# orbitals with #14# electrons.
You can have #2# electrons per orbital due to spin and the Pauli exclusion principle - no two fermions may have the same quantum numbers in the same place, so in the same orbital the two electrons must have different spins to obey Pauli exclusion. One has a #+1/2# spin and one has a #-1/2# spin.
