How much milk does a Guernsey cow produce a day?

First of all, I'd like to address the grammatical errors in this question, to the best of my ability. Milk does not come in an individual particle or object form, it is a liquid and comes in terms of volume. When asking about the amount of a certain volume from something, you need to as "How much", not "How many."

Secondly, when saying about something that occurs only once, the Word following is most often not in plural form, thus the correct phrase for this question would be "produce one time a day," or "once in a day," or "once a day."

Now, to answer this question, a Freisian cow may produce around 10 gallons of milk per milking. This maybe slightly less than the Holstein, a albeit close relative to this breed, which produces often over that amount in one milking. Naturally, though every cow is different and produces different amounts of milk each lactation, each day, or each milking.