The GM Powerglide transmission holds 4 quarts of transmission fluid with the stock transmission pan.
When adding transmission fluid to an empty transmission add 3 quarts of transmission fluid to the transmission then start the vehicle up and let it idle to suck the transmission fluid through the transmission.
Then add another quart of transmission fluid to the transmission and those 4 quarts should fill the transmission pan and transmission up.
Just be sure to not overfill your transmission with transmission fluid otherwise it can cause damage and blow out seals and your transmission pump.
But as long as you have 4 quarts of transmission fluid then that should be exactly enough to fill the Powerglide Transmission.