...it has like, 2. LOL. "PRECIOUS ONE" & "BLUE TUESDAY", i believe.
i guess "SPECIAL HAPPINESS"
could be a ballad, but i think it's more on the cute & adorable side.
i think the album with the most ballads / slow songs would be "cartoon KAT-TUN II You". it has "Bokura no Machi de", "Utai Tsuzukeru Toki"
, "MY ANGEL, YOU ARE MY ANGEL", & Maru's "key of life", as well as Kame's "someday for somebody" are ballads too.