- Have you checked the Trash bin in iPhoto?
- And checked, if the photos have been hidden accidentally? Press the command "Photos > Hidden Photos ⇧⌘H". To unhide all photos, use the command "Photos > Unhide Photos ⌘L"
If these simple methods do not increase the photo count, try to repair your iPhoto library with the First Aid Tools.
Old Toad has a nice description how to repair and rebuild in this post: Try first the repair option, and if that does not help, back up your iPhoto library and go for "Rebuild".