I hope I will not embarrass you, I am asking this question in regards to a trivia question which is, how many breaths a minute does a healthy adult take?
Optimal would be depending on the activity engaged in. Deep deep rest could be less than one per minute but as a practical matter perhaps three. Eight is good but not great. Pauses are critical. See the Breathing Test answers for more insight on pauses.
Beyond that the longer, slower, effortlessly deeper breaths taken in most any activity would seem to be superior than faster ones. I repeat effortlessly because many force the breath which force actually restricts the deepest easiest breath. Rapid Breathing Improvement