Translation:How many girlfriends does he have?
Agreed. But I'd like it better if the opposite was also presented. Sometimes it's the woman who's poly. Lol
The character combo 几个 is the simple way to ask 'how many' of a certain type of person or object.
"How many girlfriends has he got?" was rejected, this is a perfectly correct answer.
It's actually improper English to say it that way, although it's common in everyday slang
It's quite acceptable and generally taught in EFL classes alongside the other form.
You could say that in English directly same way as Chinese too but it shows surprise when emphasis is on "how many"... "he has /how/ many girlfriends?!"
'Has he' means the same as 'does he have' and is perfectly acceptable in English.