It's difficult to define precisely how large or small an 'Orc-hold' could be. A 'hold' would normally be a fortress or refuge or perhaps (especially in the case of the Orcs) a cavern, and on that basis the named examples we have would all seem to be too large and populous to qualify. If we define an Orc-hold as any place populated by Orcs, then Goblin-town and Gundabad would fall under that definition, as would Moria in its later days. On that basis, there would also have been numerous Orc-holds to be found in Mordor, and doubtless also in Dol Guldur.