Rule 702.176a

Impending is a keyword that represents four abilities. The first and second are static abilities that function while the spell with impending is on the stack. The third is a static ability that functions on the battlefield. The fourth is a triggered ability that functions on the battlefield. “Impending N—[cost]” means “You may choose to pay [cost] rather than pay this spell’s mana cost,” “If you chose to pay this spell’s impending cost, it enters with N time counters on it,” “As long as this permanent’s impending cost was paid and it has a time counter on it, it’s not a creature,” and “At the beginning of your end step, if this permanent’s impending cost was paid and it has a time counter on it, remove a time counter from it.” Casting a spell for its impending cost follows the rules for paying alternative costs in rules 601.2b and 601.2f–h.

Find this number in the current Rules