Hordewing Skaab — ruling 1

The number of cards you discard is equal to the number of opponents that were dealt combat damage by your Zombies, even if another effect changed the number of cards you drew.

Hordewing Skaab — ruling 2

If some of your Zombies have first strike but others don't, or if you have a Zombie with double strike, the ability will trigger twice, the first time counting the number of opponents who were dealt damage in the first strike combat damage step, and the second time counting the ones who were dealt damage in the second combat damage step.

Hordewing Skaab : règles, fonctionnement et interactions (MTG)

Hordewing Skaab 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

Flying Other Zombies you control have flying. Whenever one or more Zombies you control deal combat damage to one or more of your opponents, you may draw cards equal to the number of opponents dealt damage this way. If you do, discard that many cards.

How it works : Hordewing Skaab

Original English text — Wizards of the Coast / Scryfall

#1September 24, 2021· Wizards of the Coast
The number of cards you discard is equal to the number of opponents that were dealt combat damage by your Zombies, even if another effect changed the number of cards you drew.
No other card is named in this ruling.
#2September 24, 2021· Wizards of the Coast
If some of your Zombies have first strike but others don't, or if you have a Zombie with double strike, the ability will trigger twice, the first time counting the number of opponents who were dealt damage in the first strike combat damage step, and the second time counting the ones who were dealt damage in the second combat damage step.
No other card is named in this ruling.