Muxus, Goblin Grandee — ruling 1

If a card in a player's library has {X} in its mana cost, X is considered to be 0.

Muxus, Goblin Grandee — ruling 2

The bonus Muxus gets is determined only as its last ability resolves. Once that happens, the bonus won't change later in the turn even if the number of Goblins you control changes.

Muxus, Goblin Grandee : règles, fonctionnement et interactions (MTG)

Muxus, Goblin Grandee 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 Muxus enters, reveal the top six cards of your library. Put all Goblin creature cards with mana value 5 or less from among them onto the battlefield and the rest on the bottom of your library in a random order. Whenever Muxus attacks, it gets +1/+1 until end of turn for each other Goblin you control.

How it works : Muxus, Goblin Grandee

Original English text — Wizards of the Coast / Scryfall

#1June 23, 2020· Wizards of the Coast
If a card in a player's library has in its mana cost, X is considered to be 0.
No other card is named in this ruling.
#2June 23, 2020· Wizards of the Coast
The bonus Muxus gets is determined only as its last ability resolves. Once that happens, the bonus won't change later in the turn even if the number of Goblins you control changes.
No other card is named in this ruling.