Rule 702.95a

Overload is a keyword that represents two static abilities that function while the spell with overload is on the stack. Overload [cost] means “You may choose to pay [cost] rather than pay this spell’s mana cost” and “If you chose to pay this spell’s overload cost, change its text by replacing all instances of the word ‘target’ with the word ‘each.’” Casting a spell using its overload ability follows the rules for paying alternative costs in rules 601.2b and 601.2f–h.

Find this number in the current Rules