User blog comment:Emptylord/Season 3 - PBE Patch Notes/@comment-4922088-20121110013229/@comment-3974211-20121110013640

Well, once purchased, you have free wards all game (or, at least, 4 wards for free every time you recall). From what I can tell, the limit only applies to the item (well, to any item with the "Ghost Ward" passive).