Having a higher number of C ring subunits in an organism’s ATP synthase allows for more stable ATP synthesis - this is beneficial in organisms that have variable energy inputs, particularly plants (as the sun may be dimmed by a cloud, or night). More C ring subunits reduce the concentration gradient of protons required for ATP synthesis, allowing for less stable energy input. This is a similar concept to gears on a bicycle, with a larger gear allowing for less energy to be put in to each revolution of the pedals.
Larger C ring subunits give lower ATP : proton ratios, reducing the efficiency of synthesising ATP with more C ring subunits. Typically, C ring subunit numbers range from eight to 14.