Here is the situation 1 :

 

A man goes to my store to buy a cake for his parents � wedding anniversary. And he would like to write something on that cake. And I have to ask him about the number of years so that I can write it on that cake. What or how should I ask?     

Situation 2: 

My friend has some younger brothers and older brothers. I want to ask him about his position in his family. What should I ask? Can I ask "what is your place in your family?   

For your first situation, I would ask "How long have your parents been married (for)?" or "Which anniversary are the celebrating?" You could also ask "How many years have they been married (for)?"

 

The second situation seems a bit odd to me. "Which child are you?" or "Where do you come in the family order?" could work. We might more naturally ask "Are you the oldest?" or "Are you the youngest?" and then wait for the friend to clarify his position.

 

There �s no such thing as "How manyth" in English! 

"Which anniversary is it?"

 

"Which child are you?"