Rule 701.8g

A spell or ability may instruct a player to exchange two numerical values. In such an exchange, each value becomes equal to the previous value of the other. If either of those values is a life total, the affected player gains or loses the amount of life necessary to equal the other value. Replacement effects may modify this gain or loss, and triggered abilities may trigger on it. If either of those values is a power or toughness, a continuous effect is created setting that power or toughness to the other value (see rule 613.3b). This rule does not apply to spells and abilities that switch a creature’s power and toughness.

Find this number in the current Rules