Alpine Houndmaster — ruling 1

The value of X is determined only as Alpine Houndmaster's last ability resolves. Once that happens, the value of X won't change later in the turn even if the number of attacking creatures changes.

Alpine Houndmaster : règles, fonctionnement et interactions (MTG)

Alpine Houndmaster 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

When this creature enters, you may search your library for a card named Alpine Watchdog and/or a card named Igneous Cur, reveal them, put them into your hand, then shuffle. Whenever this creature attacks, it gets +X/+0 until end of turn, where X is the number of other attacking creatures.

How it works : Alpine Houndmaster

Original English text — Wizards of the Coast / Scryfall

#1June 23, 2020· Wizards of the Coast
The value of X is determined only as Alpine Houndmaster's last ability resolves. Once that happens, the value of X won't change later in the turn even if the number of attacking creatures changes.
No other card is named in this ruling.