> Does not one use up the same amount of cards
> playing for example 6 hands over 3 rounds as
> opposed to 6 hands over 6 rounds.

Of course not. If you play one spot alone against a dealer then each round consume two hands of (on average) 2.7 cards, or 5.4 cards. In this case 6 rounds will use around 32.4 cards.

If you play 2 spots then each round consumes three hands of 2.7 cards, or 8.1 cards. In this case 3 rounds will use around 24.3 cards. If you do this in a positive shoe then you still have enough cards left for at least one more round.

You must count the cards used by the dealer. Do you understand?