Karlov of the Ghost Council and multiple counters
Karlov will get 6 +1/+1 counters with Righteous Cause. This is because the trigger says "Whenever a creature attacks", so it triggers for each creature. If however it had been worded like Path of Bravery, which says "Whenever one or more creatures you control attack", that would only be one instance of life gain and thus only 2 +1/+1 counters.