The amount of quarts of transmission fluid that a th400 transmission with the stock transmission pan will hold around 11 to 12 quarts of transmission fluid.
So if the torque converter and transmission is completely empty of transmission fluid when you fill the th400 transmission then you should buy 12 quarts of transmission fluid to make sure you have enough transmission fluid to fill the transmission.
What you should do is add around 5 quarts of the transmission fluid and then start the engine and let the engine idle and keep checking the transmission fluid level.
Then add a quart of transmission fluid and check the transmission fluid level and then keep adding more transmission fluid until the transmission fluid is full.
Don't add the full 12 quarts because you might overfill it which is bad for the transmission.
After the 5 or 6 quarts of transmission fluid then just add a little at a time until it is full.