So in English "There are six cows", the word "there" isn't really doing the same thing as it is in a sentence like "Bill lives there". One way you can tell is you can just as easily say "there are six cows here", and there isn't any contradiction or anything. "There are six cows" just means "six cows exist" or "six cows are present" or something like that.
In Hindi, you don't need the "there" to express the "exist" part. It's enough to say "Six cows are", which is what this sentence says. You could say "there" by adding वहाँ, but that sort of changes the sentence slightly from what the English translation is -- it would be more like saying "Six cows are there", which definitely means something a bit different in English!