5 in total thus far, according to the very helpful
KoboBooks.com Discount Coupons thread in the device forum.
The other ones which will expire on the same date are
oct2discount20
oct2romance20
oct2youngadult20
And if you've still got money to spend after that, there's a few more no-known-expiry-date discount codes in the other thread which people might or might not have already used up.