Question: how many electrons are required to fill completely all the 4f orbitals?
Answer: Each orbital can hold 2 electrons (one of each spin), and a maximum of 7 f orbitals can have the same energy without violating Pauli's exclusion principle. The number of electrons in a 4f subshell can be anything between 0 (if it isn't filled) and 14 (2 electrons per orbital times 7 orbitals = 14 electrons).