I don't know if there is a limit. I have used 4 cards. I had one main card I used, one time my card wasn't handy so the photographer gave me another one, I got another one at the water park that stays around your wrist, and I added my sister-in-law's card to our account since it was all the same family.
There are people who share Photopass cards on DIS to get the CD cheaper and I have seen up to 10 people doing that, so I don't think there really is a limit.
If there is a limit, we haven't hit it yet. Last trip we had 16 different cards-mostly due to the ones we accumulated at the waterparks where they handed you a new photopass bracelet each time they took a pic.