In this rule

Historical Rules · 4 April 2025

Rule 306.5d

Official source text from the 4 April 2025 Rules release

Permalink

Each planeswalker has a number of loyalty abilities, which are activated abilities with loyalty symbols in their costs. Loyalty abilities follow special rules: A player may activate a loyalty ability of a permanent they control any time they have priority and the stack is empty during a main phase of their turn, but only if none of that permanent’s loyalty abilities have been activated that turn. See rule 606, “Loyalty Abilities.”

Find this number in the current Rules