It's '5x' for two reasons: (a) that's a reasonable target for how many times you might have to ask it before you hit a root cause and (b) the 'five' is meant in

It's '5x' for two reasons: (a) that's a reasonable target for how many times you might have to ask it before you hit a root cause and (b) the 'five' is meant in a more metaphorical sense - you're meant to ask until you hit the root cause, but 'five' is catchier.