His 20-30 quarts comes from the fact that we don't really drain the torque converter when we drop the pan. Both times that I dropped the pan, replaced the filter/gasket, and a refill, I only used about 6 quarts. With a total system capacity of 8.9L, that means we have about 3.5 quarts of the original fluid left (in the torque converter).
Do do it "properly", you'd need to do the entire drain/fill procedure several times to cycle all of the new fluid through the torque converter. But since you're mixing new and old fluid, it's never going to be all brand new.
Assuming a thorough mix of new and old between drains:
1st drain/fill: 3.5qts old, 6qts new (63% new) - 6qts used
2nd drain/fill: 1.3qts old, 8.2qts new (86% new) - 6qts used
3rd drain/fill: .5qts old, 9.1qts new (96% new) - 6qts used
4th drain/fill: .2qts old, 9.3qts new (98% new) - 6qts used
So...to get ~98% new fluid, you'd need to use 24qts.
However, almost everyone is going to do just a single drain/fill, which will get about 2/3 of our fluid replaced. Even for the most diehard folks, it doesn't make any sense to do more than 2 drain/fills.
......Sparkling Graphite '05 330Ci ZHP w/ NAV......