Quirion Beastcaller — ruling 1

You choose the targets and announce how the +1/+1 counters will be distributed as you put Quirion Beaastcaller's second ability on the stack. Each target must receive at least one +1/+1 counter.

Quirion Beastcaller — ruling 2

If some of the creatures are illegal targets as the second triggered ability tries to resolve, the original distribution of counters still applies and the counters that would have been put on illegal targets are lost.

Quirion Beastcaller : règles, fonctionnement et interactions (MTG)

Quirion Beastcaller 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

Whenever you cast a creature spell, put a +1/+1 counter on this creature. When this creature dies, distribute X +1/+1 counters among any number of target creatures you control, where X is the number of +1/+1 counters on this creature.

How it works : Quirion Beastcaller

Original English text — Wizards of the Coast / Scryfall

#1September 9, 2022· Wizards of the Coast
You choose the targets and announce how the +1/+1 counters will be distributed as you put Quirion Beaastcaller's second ability on the stack. Each target must receive at least one +1/+1 counter.
No other card is named in this ruling.
#2September 9, 2022· Wizards of the Coast
If some of the creatures are illegal targets as the second triggered ability tries to resolve, the original distribution of counters still applies and the counters that would have been put on illegal targets are lost.
No other card is named in this ruling.