One of the challenges (I won't say problems) with the Drake Equation is we have some sense for some numbers and they're very very large. Which encourages people to use formulations for some of the other terms like "even if we assume this is one in a billion" and this is "one in a million," you still come up with a significant number of civilizations. When, in fact, some other term may be so vanishingly small that it's only happened once. (Or there may be something else, e.g. a great barrier, that's not being taken into account.)

But, as others have written, it's not really intended to be used that way.