Nykthos Paragon — ruling 1

"Do this only once each turn" refers to putting +1/+1 counters on your creatures using the triggered ability. As long as you haven't yet chosen to put +1/+1 counters on your creatures with it, all instances of gaining life will cause Nykthos Paragon's ability to trigger.

Nykthos Paragon — ruling 2

"Do this only once each turn" refers only to the ability it is attached to, not other instances of the same ability. In other words, if you control multiple Nykthos Paragons, you will be able to do this once for each of them.

Nykthos Paragon — ruling 3

Once you have chosen to put +1/+1 counters on your creatures, further instances of gaining life will not cause the ability to trigger.

Nykthos Paragon — ruling 4

If multiple instances of the ability are on the stack, you will be able to put +1/+1 counters for only one of those instances. Once you do that, other instances will do nothing as they resolve.

Nykthos Paragon — ruling 5

If a creature you control is dealt lethal damage at the same time that you gain life, it won't receive +1/+1 counters from Nykthos Paragon's ability in time to save it.

Nykthos Paragon — ruling 6

Each creature with lifelink dealing combat damage causes a separate life-gaining event. For example, if two creatures you control with lifelink deal combat damage at the same time and you haven't used it yet, Nykthos Paragon's ability will trigger twice. However, if a single creature you control with lifelink deals combat damage to multiple creatures, players, and/or planeswalkers at the same time (perhaps because it has trample or was blocked by more than one creature), the ability will trigger only once.

Nykthos Paragon : règles, fonctionnement et interactions (MTG)

Nykthos Paragon 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 gain life, you may put that many +1/+1 counters on each creature you control. Do this only once each turn.

How it works : Nykthos Paragon

Original English text — Wizards of the Coast / Scryfall

#1June 18, 2021· Wizards of the Coast
"Do this only once each turn" refers to putting +1/+1 counters on your creatures using the triggered ability. As long as you haven't yet chosen to put +1/+1 counters on your creatures with it, all instances of gaining life will cause Nykthos Paragon's ability to trigger.
No other card is named in this ruling.
#2June 18, 2021· Wizards of the Coast
"Do this only once each turn" refers only to the ability it is attached to, not other instances of the same ability. In other words, if you control multiple Nykthos Paragons, you will be able to do this once for each of them.
No other card is named in this ruling.
#3June 18, 2021· Wizards of the Coast
Once you have chosen to put +1/+1 counters on your creatures, further instances of gaining life will not cause the ability to trigger.
No other card is named in this ruling.
#4June 18, 2021· Wizards of the Coast
If multiple instances of the ability are on the stack, you will be able to put +1/+1 counters for only one of those instances. Once you do that, other instances will do nothing as they resolve.
No other card is named in this ruling.
#5June 18, 2021· Wizards of the Coast
If a creature you control is dealt lethal damage at the same time that you gain life, it won't receive +1/+1 counters from Nykthos Paragon's ability in time to save it.
No other card is named in this ruling.
#6June 18, 2021· Wizards of the Coast
Each creature with lifelink dealing combat damage causes a separate life-gaining event. For example, if two creatures you control with lifelink deal combat damage at the same time and you haven't used it yet, Nykthos Paragon's ability will trigger twice. However, if a single creature you control with lifelink deals combat damage to multiple creatures, players, and/or planeswalkers at the same time (perhaps because it has trample or was blocked by more than one creature), the ability will trigger only once.
No other card is named in this ruling.