How many red-eared slider turtles can live in a 20 long?

Only baby RES can be housed in a tank that small. But, for many you can keep in there, I'd say two baby RES. But, do realize that you will need a bigger tank, because a full grown male will need a 75 gal, and a full grown female will need at least a 120 gal. If you can't afford this you can always get a stock tank or a plastic container bigger than 55 gallons.