There's no limit other than all the punches being used up. Speaking of which, don't buy the card until on board and you ask the barista how many punches per drink of the kind you will order.
The only (not so meaningful) discount you get is if you book 8 cabins. Keep watching the inventory over the next few months - that will drive the price. Cruiseplum provides a nice historical price trend.
The package is offered at a discount because it gives RCI flexibility to give you whatever times are left over on board AFTER folks paying full price pick times. If you want the certainty of a specific time, you need to pay full price.