Jetzt schlägt es aber 13 - also possible: Aber jetzt schlägt es 13 (this is more emotional)

Word by word translation: "but now it hits 13"

figurative meaning of: The church clock strikes at most 12 times, never 13 times.
Every clock that chimes on the hour, strikes one beat for every full hour. There is a maximum of 12 o'clock. 13 o'clock does not exist. So there should never be 13 strokes.

Usage:
To say "now it strikes 13" means that someone - just now suddenly - has said or has done something that normally would never happen or should not happen, something absurd, unjustified or something outrageous or impertinent.

It expresses that the person has gone too far and the measure is full. It is meant to set a limit.
It is said directly to the person who was just doing it. It is an immediate response.
It's a direct, spontaneous emotional reaction, a direct answer.
You don't use it when you tell the incident to another person.