In the "olden" days, the rule was to put a comma after ANY element that came at the beginning of the sentence. Ah, the good old days!
The rule today that many people get confused about is the rule about putting a comma after a prepositional phrase at the beginning of the sentence. It is often stated as something like "Put a comma after a prepositional phrase at the beginning of the sentence that has four words or more."
This does not take into account parentheticals and transitional elements and independent comments, for which this rule does not apply.
...By the way, he was not interested in that.
...In contrast, he was not interested in that.
...To my knowledge, he was not interested in that.
The rule should read something like
"When one prepositional phrase that is a simple modifier comes at the beginning of the sentence, put a comma after it if it is 'long.' 'Long' is somewhere between four and five words.'"
...On a good day we were able to do about 20 of them.
...During the organizational meeting, we were able to do....
I believe that counting these words and sticking to the exact number of words misses the point. The point is you need to slow the reader down (with a comma) to show that the prepositional phrase has concluded and that the subject and verb is coming up.
Happy punctuating!
Margie