If we think of electron configurations as an electron's "address". The s, p, d, and f tell us the type of house they live in and refer to the shape of the orbital where we find a particular electron.
Each orbital, no matter what the type, can only hold 2 electrons. When we look in any one principal quantum shell (i.e. n=3), we can have multiples of some orbital types. There is only 1 s orbital in each shell, but there are three p orbitals, 5 d-orbitals and 7-f orbitals. We don't have any d orbitals until the n=3 level and no f-orbitals until n=4