Multiple Apple TV connections to one iTunes Library
I don't see any reason why there would be a programmed limit since OS X server can serve hundreds of computers from the same share and to intentionally break the n'th connection would be poor form to program if there weren't a way to set that limit.

Photo sharing is very low bandwidth compared to HD video streaming, so you should not have any problem with 16 large images - even if they change once a second since a video stream is basically 24 fps and I've had more than three Apple TV stream from the same library on a MacBook Air without running into network bandwidth or IO contention issues.