You would ask your friend how many guests he invited to his engagement party.Context: I asked my friend how many guests he invited on/to his engagement party?
PS: Consider engagement as wedding party.
The right preposition in English is "to", not "on".I am confused with the reply in thread 16.
Consider this sentence...
Sam, you know I invited William, John and Liza on my engagement.
In my language it looks on is better , but I don't know about the right preposition.