Z, or the number of formula units, depends on the symmetry of the structure. By doping your material you change the symmetry and therefore the unit cell. You can't really calculate it, you have to know the crystallographic structure of your material.

In your examples, a single unit cell of LaFeO3 contains 4 Lanthanum ions, 4 Iron ions and 12 Oxygen ions. When you dope it with Calcium, the unit cell is defined differently (because you try to keep the unit cell as small as possible) and only contains 2 Lanthanum/Calcium ions (these sites are shared), 2 Iron ions and 6 Oxygen ions.

Z corresponds to the lowest multiplicity out of all occupied Wyckoff sites in the respective unit cell and is therefore closely related to the space group symmetry as Michael Patrick Mueller mentioned.

In oP20 (or Pnma, spgr #58) four different Wyckoff sites exist: 4a, 4b, 4c, and 8d. Z = 4 because one or more Wyckoff sites with multiplicity 4 are occupied.

In oI10 (or Immm, spgr #71) there are 15 different Wyckoff sites ranging from 2a to 16o. If one of the four Wyckoff sites with the multiplicity 2 is occupied, Z=2.

Please note that this technically also changes the chemical formula from ABX3 to A2BB'X6, where you have two crystallographically different Fe-ions. If you used two different transition metal cations, they would share the 4a site in oP20 in a disordered fashion, but in oI10, those two cations could occupy two different sites with perfect order (see Sr2WFeO6 for example).