nonkosher milk mixed with a large quantity of kosher milk, is the milk kosher?

Thank you for your question, If the quantity of the non-kosher milk is less than a sixtieth of the kosher milk, it is kosher and one can drink it. If one purposely mixed the non-kosher milk with the kosher milk, and schemed it this way to allow it to be drunk by Jewish people, than it may be forbidden for the person who mixed it together to benefit from this milk(see Shulchan aruch 99;6). By the way, some poskim permit drinking milk that was milked by a non-Jew, providing that there was government supervision (Chazon Ish, Igrot Moshe). Therefore, if your question refers to to a situation where non-Jews milked the cows, I believe that ex post facto, we can definitely rely on these poskim and permit the milk. All the best!