Legolas, Counter of Kills — ruling 1

"Do this only once each turn" lets you choose whether or not to untap Legolas as the triggered ability resolves. If you don't untap it, the ability will trigger again the next time the condition is met. Once you choose to do so, the ability will no longer trigger for the rest of the turn.

Legolas, Counter of Kills : règles, fonctionnement et interactions (MTG)

Legolas, Counter of Kills is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Reach Whenever you scry, if Legolas is tapped, you may untap it. Do this only once each turn. Whenever a creature an opponent controls dies, put a +1/+1 counter on Legolas.

How it works : Legolas, Counter of Kills

Original English text — Wizards of the Coast / Scryfall

#1June 16, 2023· Wizards of the Coast
"Do this only once each turn" lets you choose whether or not to untap Legolas as the triggered ability resolves. If you don't untap it, the ability will trigger again the next time the condition is met. Once you choose to do so, the ability will no longer trigger for the rest of the turn.
No other card is named in this ruling.