I don't know. I think they keep the number vague on purpose to keep people from gaming the system.
I do know it has to be a pattern of behavior because everybody has off days and bad games. Riot takes the stance that for the most part you shouldn't be punished for that off day or bad game, it's only when you show a pattern of behavior that you get punished.
My only input on the matter is that I probably would've fixed my router issues earlier if LeaverBuster hadn't given me so many chances. But there are plenty of people who feel like LeaverBuster is too restrictive. Riot has decided on a number that they feel is fair and balanced and we have to trust that their millions of games of data trump our personal experiences. Whichever way we fall on our views of their punishment system.