this question makes no sense in English because a millenium is constant. a more natural question would be How many years does a millenium have?
The question makes sense to me, but I agree that "How many years does each millennium have?" is more natural in English.
chữ "kỷ" trong thiên niên kỷ cũng có thể là "kỉ" mà
Both answers are accepted.
I wrote kỉ and it's still not accepted
how many years are there in a millenium?
How many years in a millenium? Should also be accepted. It's not a literal translation, but it IS more common in English.