"How many friends have you?" probably wasn't accept because it makes no sense in American English. I understand it's common in the UK, but here that sounds completely as unnatural as
"Ate you a sandwich?"
We always say, "do you have..."
It's just a pity that Duolingo so often rejects English sentences that are considered correct in British English but sound wrong to Americans. One hopes that enough "my answer should be accepted" reports will make these acceptable. Can't be easy to please everyone all the time, but I don't think its Duolingo's job to correct my English.
They get thousands of suggestions each day. It takes time.
IMHO We are here to learn Spanish, not to polish our English. And therefore, the meaning and context of the translation from Spanish are supposed to take priority over grammatical and spelling errors in English. Thus, the answers in English (NOT in Spanish), which are making sense but are not right just "technically/grammatically", should be accepted with the correcting comment (sort of "You have a typo in your answer...") and such answers should not be counted wrong. Otherwise, instead of concentrating on Spanish, we are getting sucked into the time consuming forum discussions about usage of English.
It's not grammatically correct English. Putting "have you" after "friends" makes it sound like an unfinished sentence in the present perfect tense, like "how many friends have you had?"