The lowest energy level that can have an f orbital is the fourth energy level. It is large enough to accommodate the 14 f orbital electrons in addition to the s, p and d orbital electrons.
Each energy level has a limit to the maximum number of electrons it can accommodate, as defined by the formula 2n^2, where n is the principal quantum number. The first energy level can have two electrons, the second can have eight, the third can have 16 and the fourth can have 32. The electrons in each energy level are further subdivided into (in increasing order of energy) the s, p, d and f orbitals. The s orbital can hold two electrons, p can hold six, d can hold 10 and f can hold 14.