If you want to know how many sons someone's father has in english you would ask him : :"How many brothers do you have ?" This is the correct way to know how many sons his father has , but the other expression is not correct in english !

It's not correct to ask that; "Manyth" is not actually an english word. You could ask "What son are you? The first, the second...", or "What number son are you?"

What is your birth order? Does this sounds good!?