Reduplication to indicate pluralisation can be used for clarification, but it's not always necessary. Here it's obvious that there is more than one book, because there are six of them, so no need to use buku-buku. I'm not a native speaker, but I think "enam buku-buku" should be an accepted response though.
but I think "enam buku-buku" should be an accepted response though.
No, that is not accepted.
Reduplication is not used when there is already a number (or another word) indicating the quantity:
"Satu buku" = One book.
"Dua buku" = Two books.
"Tiga buku" = Three books.
"Banyak buku" = Many books.
"Sedikit buku" = Few books.