Number of orbitals in full configuration interaction
In simple terms, Full CI uses all of the possible orbitals (a "full" active space). Thus the number of active orbitals should equal the size of your basis set. If you have 10 basis functions, and then perform a Hartree-Fock calculation, you get 10 orbitals out (some occupied, some virtual).

Complete Active Space (CAS) methods, on the other hand, only use some of the orbitals and then perform a Full CI on that subset of orbitals, leaving the other orbitals (inactive space) alone.

Full CI is CAS in the total orbital / total active space limit.