Rule 702.77a

Conspire is a keyword that represents two abilities. The first is a static ability that functions while the spell with conspire is on the stack. The second is a triggered ability that functions while the spell with conspire is on the stack. “Conspire” means “As an additional cost to cast this spell, you may tap two untapped creatures you control that each share a color with it” and “When you cast this spell, if its conspire cost was paid, copy it. If the spell has any targets, you may choose new targets for the copy.” Paying a spell’s conspire cost follows the rules for paying additional costs in rules 601.2b and 601.2f–h.

Find this number in the current Rules