It is annoyingly ambiguous. When I am at work and a customer requests "a couple of" something, I have to ask whether they literally mean two, because sometimes they want more than two. I avoid using the expression when the exact amount is important. When I say it, I mean "about two to five, it doesn't matter how many". For instance: "I wondered if anybody was home, so I knocked on the door a couple times." It doesn't matter exactly how many times I knocked, just that there was some brief knocking.