That completely depends on the size of the pool, and on length of time
the evaporation continues.
If you're talking about the amount of water that evaporates from a
kiddie pool in the next half hour, then cups or quarts might producer
a convenient number.
If, on the other hand, you're willing to wait longer, then the day will
eventually arrive when an entire olympic-size pool will go bone-dry.
If it's precision you're concerned with, then smaller units are typically
used for measurements of greater precision.