there are some cases of username who has 20++ word on it
2007 account
same like above, an account from 2007
pretty recent account from 2015
Whoops, this is the most recent one: 2018

I am guessing 12 is the max.
As for why, this is just an educated guess from me. The word restriction for username may got shortened over time to reduce the annoyance of that longass name