In most of time when introducing people, I heard "He is friend of mine" instead of "He is my friend." Could you tell me why so?
'He is one of my friends' makes it clear that I have many other friends.
Having said that, it's also common to hear 'He is my friend' when he's not the only friend. It suggests that the friend is special and the friendship is important to the speaker.
A shorter answer to your question would have been 'You can please yourself how you introduce a friend'.