The question is very simple: how many kanji there are that have a 'unique' ON-reading, not shared by any other kanji? Or, as it is probably a too general question given the variety of kanji: how many one-kanji ON-readings exist in the Joyo list? Do any of those readings gain additional kanji if we start to consider Jinmeiyo-kanji / non-Joyo-kanji / non-standard readings? Tricky readings like [頁]{ページ} or [糎]{センチメートル} are excluded.
Example, which I know: the チャ reading is only with 茶 while we are in Joyo, but if we extend our search, there are at least 茗 and 楪 which share the same.