Bag End Porter — ruling 1

The value of X is determined only as Bag End Porter's triggered ability resolves. It won't change later in the turn if the number of legendary creatures you control changes.

Bag End Porter : règles, fonctionnement et interactions (MTG)

Bag End Porter 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 this creature attacks, it gets +X/+X until end of turn, where X is the number of legendary creatures you control.

How it works : Bag End Porter

Original English text — Wizards of the Coast / Scryfall

#1June 16, 2023· Wizards of the Coast
The value of X is determined only as Bag End Porter's triggered ability resolves. It won't change later in the turn if the number of legendary creatures you control changes.
No other card is named in this ruling.