There is no limit, BUT and this is a big BUT, make sure you purchase the card ahead of the first person getting a single photo taken in order to get the $99 discounted Photopass CDs. If you (and the group) wait to purchase with the discounted code, you will end up paying $50 more than necessary.
It used to be you could wait until after a trip to purchase with the discounted code, but that is no longer the case. That loophole has been closed.
Hope everyone has a magical time!