How many gallons of water are in a junior Olympic size swimming pool?

minimum 2,500 m3 (88,000 cu ft) or

2,500,000 L (550,000 imp gal; 660,000 US gal) or

138,888,888.889 mol,

depending on depth

Note only minimum is specified, depth can be greater and so can gallons as long as it meets minimum requirement.