The size of the c ring in the Fo rotor allows for ATP synthesis from H+ gradients. Fewer c rings in the Fo rotor requires a greater proton gradient to be formed, due to the higher torque required to rotate the central stalk and allow the necessary conformational changes to occur in the F1 catalytic subunit. Animals have lower numbers of C subunits (down to 8), due to the more stable supply of food they have access to.
Plants tend to have more C subunits (up to 14), reducing the H+ gradient required to rotate the central stalk and allow ATP synthesis to occur. More subunits allow for ATP synthesis even when the energy input is not stable (darkness, …), enabling the plant to continue living through the night etc… A higher number of C subunits increases the ratio of H+ to ATP, as 3 ATPs will be produced for every rotation of the Fo rotor - increasing the requirement for H+ ions to be transported to synthesise the same quantity of ATP.