Multiple photopass card numbers will be combined on a shared online Photopass account. You can have an unlimited number of photopass cards entered on an account, and you can have an unlimited number of photos when you order the CD's. I personally just ordered the CD with over 1900 pictures on our account and received 7 CD's. Disney puts 300 pictures on each CD, but that really doesn't matter because they'll send as many CD's as it takes to get all the pictures on a single online account.
As for whether you'd receive pictures of other families... That depends on how the organizer does it. Depending on how easy it is to identify and separate the different family groups, you might just get yours, or you might get some pics of other families also. It should be easy enough for anyone to copy all the pics to their PC and then delete the ones that aren't of their family, so I don't see that it would be an issue at all.
Hope this answers your questions!