Rain barrels will typically capture 50 to 60 gallons. A small 8' x 10' area of roof will generate 50 gallons of water during a 1 inch of rain.
To calculate the gallons of water that is generated from 1 inch rainfall:
measure the square footage of roof that drains to the downspout divide that number by 43,560 (converting it to % of acre) and then multiply by 27,152 gallons (the amount of water that falls on an acre from a one inch rain.)
Your rooftop is 25 foot long by 20 foot wide, which is 500 square feet.
500 S.F. / 43,560 S.F. per acre = 0.0115 acres draining to downspout 0.0115 acres x 27,152 gals per acre per inch of rain = 312 gallons of runoff to your downspout per inch of rain.
Each inch rainfall will deliver 312 gallons of runoff to your rain barrel. Meaning it would take more than 6 50 gallon rainbarrels to capture all the rainfall draining to that one downspout.
If you have less than six barrels, be sure to have an overflow pipe that moves water away from your foundation and, ideally, to a rain garden that will help infiltrate the water into the soil.