It seems to me like the absolute cutoff is something like party size +1 number of forced spirit guardian applications based on maximum teamwork efforts. That’s actually not as good as the whole party being clerics and each just independently ready action reaction move when an enemy moves or attacks. Providing the whole party is clerics they can apply spirit guardians 2x party size (provided the number of enemies is equal to or greater than the number of PCs).
*off turn movement abilities nearly always if not always take a reaction.
My personal subjective sense of when it becomes degenerate is long before party size + 1.

But even if it does less total damage, I personally feel moving the Cleric around off-turn is more degenerate than multiple Clerics, just due to the fact that multiple Clerics are actually paying for their damage with multiple spell slots.
Note, though, that depending on the method, moving the Cleric might not be less damage than multiple Clerics. One needs to include the value of the rest of the party's own attacks, since with sequentially-controlled-mount (or vehicle) shenanigans moving the Cleric around doesn't require the other party member's actions.
Of course one can go even father by
combining actionless off-turn Cleric movement with your multiple-Cleric concept, where each Cleric on their turn hits the enemies with two copies of
Spirit Guardians (from themselves and whomever is passenger or riding shotgun), plus an action spell of their own.
Edit: or with a big wagon maybe n Clerics can hit the enemies with n copies of Spirit Guardians on each of their n turns, plus their own actions, simply by rotating drivers using movement to get to the driver position and item interaction to take the reins? That would be n^2 Spirit Guardians activations per round per enemy. If each cleric readies actions to move on and off the wagon and the enemies' geometry is absolutely perfect, one should be able to get up to n^2+n activations per round per enemy.