Hi,
1. as @holch,
in your analysis tool (like SPSS) you may calculate it
Here an SPSS example
data (M1 was only displayed, if S1==1)
result
Of course, SPSS can't distinguish between an empty response because of "not displayed" or because of "the respondend didn't select any"
But usually these questions are mandatory with an exclusive answer "Don't know / no answer"
2. Just have a look at the question of the condition. Here you see how many rerspondents had to answer the next question. (marked "blue")
3. You see the option "not displayed".
And you see the total count. So the difference is the number of respondents who had to answer this question. (marked "red")
Joffm