Number of Islands is counted on resolution and not on announcement.

Typhoon : règles, fonctionnement et interactions (MTG)

Typhoon 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

Typhoon deals damage to each opponent equal to the number of Islands that player controls.

How it works : Typhoon

Original English text — Wizards of the Coast / Scryfall

#1October 4, 2004· Wizards of the Coast
Number of Islands is counted on resolution and not on announcement.
No other card is named in this ruling.

Interactions with other cards

These cards' text mentions Typhoon.

Dragon Typhoon

Whenever you cast a Dragon or noncreature spell, draft a card from Dragon Typhoon's spellbook and put it onto the battlefield. Channel — {2}{R}{R}, Discard this card: Create a 4/4 red Dragon creature token with flying.