Rainbow Dash bites Applejack’s tail once, in Fall Weather Friends. Right after Applejack bit her tail earlier during the race. Applejack bites Rainbow Dash’s tail 9 times in all of FiM.
There should be an episode where the roles reverse. Meaning the Rainbow Dash bites Applejack’s tail instead of the other way around. (The fall weather friends do not count due to AJ tail bite)
You kept count?