A powerglide transmission with a deep pan holds around 6 quarts of transmission fluid if the pan is completely empty.
So have on hand at least 6 quarts and add 4 quarts to the transmission first and then after you let the engine run and bring the transmission up to temperature and let the fluid flow through the transmission add the 5th quart and recheck the transmission fluid level.
Then if it needs more slowly add the 6th quart until it's full as it may or may not take the full 6 quarts depending on if the torque converter already has some transmission fluid in it as well as the transmission lines.
But I would buy 6 quarts of transmission fluid and just add a little at a time until it's full to avoid overfilling the transmission which can cause damage.
A powerglide transmission with the stock pan holds around 4 quarts of transmission fluid while the deeper pan on the powerglide transmission holds around 5 to 6 quarts depending on if there is or isn't already transmission fluid in the transmission lines and torque converter.