Rule 702.170d

A plotted card’s owner may cast it from exile without paying its mana cost during their main phase while the stack is empty during any turn after the turn in which it became plotted. Casting a spell this way follows the rules for paying alternative costs in rules 601.2b and 601.2f–h. A plotted card may be cast this way even if it doesn’t have the plot ability while in exile.

Find this number in the current Rules