For the "regular" photostream:
"Photo Stream can contain up to 1000 photos. When you add a new photo to Photo Stream after you reach the 1000-photo limit, the oldest photo is removed."
Apple Phone Support did not know the limit for the new "shared photostream" when I contacted them, and told me they had no way to find out.
No way to find out?
They should check there own docs
"Photos in shared photo streams are not deleted after thirty (30) days, but the size of any given photo stream may be subject to photo or recipient number, and/or size limitations. Downloaded photos can be manually saved to the iOS Camera Roll."
Not exaclty the answer but closer.
I just came again across the Photo Stream FAQ: http://support.apple.com/kb/HT4486
Unlike My Photo Stream, shared photo streams are not rolling collections. Any new photos that you add to a shared photo stream do not replace your older photos. A shared photo stream can hold a maximum of 1,000 photos. When you reach your limit, you must delete some photos before adding new ones.
I wonder if the pictures in the Shared Photo Stream count against your "regular" Photo Stream?
No. Photos uploaded to My Photo Stream or Shared Photo Streams do not count against your iCloud storage.
It really would help if the information about "Shared Photo Streams" were to be found in the "Shared PhotoStream" documents and not only in the gereral "My Photo Stream" articles.