I just started to learn Go. I still have some problems to properly understand how to count territory when the game has finished (by both players passing). For example, if the game ends in the following situation, what part of the upper area would count towards black's territory?

$$c13 
$$ -------------
$$|...........X.
$$|...........XX
$$|..XXXXX....X.
$$|..X.X.XXXXWXX
$$|XXXXXXX...O..
$$|..........OOO
$$|..........OO.
$$|OOOOOOOOOOO.O
$$|..........OOO

I know, usually black would just place a stone at L11 befor finally passing but what happens if black forgets to do so? (and white also misses the opportunity to exploit this)

The resources I read about counting did not answer this question for me.