Rachel,
When you buy the PhotoPass CD, you don't receive it at that moment. Rather, it is mailed to you after your trip and after you have viewed and edited all the photos you would like to tweak.
Generally you come home with your PhotoPass card, enter the website, and do your editing and ordering from there. The CD is then mailed you to and it takes 1-2 weeks to arrive. However. if you pre-order the PhotoPass CD you can save 20%. Just follow this link:
PhotoPass Pre-Order You will not receive the CD before your trip, so no need to worry about waiting for the disc to arrive. This is just a way to save some money off the purchase price by buying in advance. You still view and edit the photos after you return from your trip. Then, when you are satisfied with your pictures you complete the order and the CD is mailed to you. It's very easy and very convenient.
You can have as many pictures taken as you would like. When we visit with extended family we generally get pictures as a group, then just with immediate family only, and sometimes the kids by themselves.
Have a great vacation!