Sudden Insight — ruling 1

If a card in your graveyard has {X} in its mana cost, X is 0 when determining that card's mana value.

Sudden Insight — ruling 2

Although land cards aren't considered, nonland cards with mana value 0 are. For example if the cards in your graveyard are an artifact card with mana cost {X} and a sorcery card with mana cost {2}{U}, you'll draw two cards.

Sudden Insight : règles, fonctionnement et interactions (MTG)

Sudden Insight 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

Draw a card for each different mana value among nonland cards in your graveyard.

How it works : Sudden Insight

Original English text — Wizards of the Coast / Scryfall

#1July 23, 2021· Wizards of the Coast
If a card in your graveyard has in its mana cost, X is 0 when determining that card's mana value.
No other card is named in this ruling.
#2July 23, 2021· Wizards of the Coast
Although land cards aren't considered, nonland cards with mana value 0 are. For example if the cards in your graveyard are an artifact card with mana cost and a sorcery card with mana cost , you'll draw two cards.
No other card is named in this ruling.