Or possibly the opposite. Cows require a lot of work to feed, especially in the colder months so I suspect millions would potentially starve. The cow population would depend on how much land they could access, whether bulls were introduced by humans to cow herds and whether humans intervened in any other way - famine or disease control, for example. If cows were left to totally fend for themselves, I think lots of them would die and their overall numbers would reduce.