Translation:How many sons do you have?
"How many sons have you?" is incorrect.
"Have you..." is not the correct way to ask possession related questions. The correct way is "Do you have...".
"Have you.." is used to ask yes/no questions like "Have you done your homework?", "Have you ever visited Disneyland?", "Have you decided what will you do tomorrow?" Etc.
I find the english translation including the word "guys" to be awkward since it "guys' generally refers to males, so to me it's kinda like asking two men how now many sons do you have. I think most people are forgetting that the word "you" is also the proper word for "you" and for 你们. My two cents worth.
So, I’ve searched that《几》and《多少》are both used to indicate questions regarding quantity. In this case, is it possible to use《多少》instead of《几》, like《他们有多少儿子？》because I think this question (the latter) is just as acceptable, just that this lesson talks about indicating quantity using《几》. But, of course, I’m still only a starter in Chinese, so I want to be sure.
I have contemplated! In the sentence "How many sons have you?" the word "you" remains the subject of the sentence. It would be equally correct to say " How many sons have I?" where "I" is the subject. It would be meaningless to say "How many sons have me?" where "me" is the object form of "I". If you read it as though "sons" were the subject and "you" were the object, of course it is meaningless. "How many sons have you?", "How many sons do you have?" and "How many sons have you got?" are all correct British English depending on the context. What I want to understand is whether there are equivalent subtleties in Chinese.
Should one have any aversion to asking mainland Chinese about the number of children they have?
No; in English "how many" is used for countable things that can be identified with a number only (I have one son, I have two sons) and "how much" is used for uncountable things that require a measure word ("How much rice do you have?" "I have a cup of rice." / "How much medicine did you take?" "I took a teaspoonful of medicine.")