In a game with 3 players, we revealed an Athelas. To Guarded this Athelas we reveal another card. I know that this other card (enemy, location or treachery) does not count in "a card per player".

But that other card was another Athelas. We added in the staging area and revealed two cards to guarded them.

Does this second Athelas count in "a card per player"? (Since it was not attached at the first)

"Guarded" acts like "Surge." You are revealing another card as the result of a card effect. So no, it doesn't satisfy the "one per player" basic game mechanic. You'll reveal Athelas, then reveal a card to guard it (which turns out to be Athelas). Then you reveal another card to Guard that first Athelas, then a card to Guard the 2nd Athelas. Then you've still got 2 more reveals to go, in a 3-player game.