From Moderrn Blackjack

We need to make a rough estimate of the number of decks that have been dealt by looking at the discard tray. If we are in the first deck, all indexes will be +1. This includes insurance and all playing indexes. If we are in the second deck, all indexes are +2. We continue this methodology throughout the shoe. If we are lucky enough to be playing the sixth deck, all indexes are +6.
What about 8 decks ?