Then why cards like Necro Gardna do not have "once per turn" in their effect?
You can have 2 Necro Gradna's in your graveyard and use them twice per turn, that's justifiable but Ancient White Stone effect is NOT the same as Necro Gardna
Necro Gardna does banish itself as the cost for its effect, preventing the same copy from being activated again immediately. You can still activate it again on the same turn, if you return it to the Graveyard somehow.
Ancient White Stone's summon effect just summons, it doesn't banish itself and just stays in the graveyard. If it wouldn't have a once per turn clause, a single copy could just summon all the Blue-Eyes monsters from your Deck during the end phase. However, since it is worded "Once per turn:" and not "This effect of xy can only be used once per turn" (like the add to hand effect), you can still use multiple copies, if all of them were sent to the graveyard that turn.
Edited by user
| Reason: Not specified