I think the English translation is technically correct, it just feels a bit awkward. Wouldn't you ask, "how many brothers do they have?" or something more along those lines?
I wouldn't even say technically correct. It's something you might use poetically, but never outside of that.
Saudara can mean (generic) sibling, yes? I reported it but am uncertain if, without further qualification (as in "saudara perempuan"), brother is assumed.