How many rounds should a swiss tournament have?

[Feijoa]

Here is my theory about it:

How many rounds in a tournament?

I looked around some more and the tournaments I can find seem to be consistent with the formula floor((P+2*7)/5) limited to a maximum of 8 rounds. P=4, 3 rounds (posted before) P=11, 5 rounds P=20, 6 rounds P=21, 7 rounds P=29, 8 rounds P=36, 8 rounds (maybe 8 is the max?) This formula predicts that P=21 would be the first tournament with 7 rounds, and P=26 would be where they reach the max of 8 rounds. EDIT: found one with P=21, and it worked! (From a three-year-old question here.)

Note that the max of 8 turned out to not be true. As far as I know they are unlimited.