Poppa,
Even according to Rav Shachter you can mix milk from two cows together according to R’Yehuda. He only holds there is a problem when statistically speaking it is more likely than not that there is some milk from a treif cow in the mix. I haven’t run the math, but I presume that if 1/10 cows is treif, then if you have a mixture from 10 cows there is a strong rubba d’leisa kamman that this mixture contains treif milk.
It isn’t shver to me that the Gemara would not discuss such a case if it was uncommon in those times to mix the milk of many different cows.