Depends on pan size and converter, deep pan and large diameter converter could take 10 or 11, shallow pan and small converter could be down around 7. I always start with 6, if just running at idle no harm will come if it's a little low, just as soon as it starts remember it will fill the conveyer first so it will leave the pan dry till you get a few more qts in it
Chassis manual says the capacity of the THM400 is approximately 23 pints. That is 12 quarts. If you have a smaller converter, it will take less. You should have poured at least 1 or 2 quarts into the converter before pushing it onto the input shaft.