Do you have any other Apple products that share your photo stream? If so, verify that those devices contain the same amount of photos. If, for example, your iPad contains 12 photos in photo stream and your iPhone contains 5, iCloud could be trying to work out the sync between the two. If you do not have any other device sharing your photostream, either disable photostream or logon to iCloud via a computer and reset photostream. Of course do that, AFTER you've saved/backed up your photos...