If one parameter of drakes equation is 0, then the whole equation is 0, because it is just a series of multiplications. And we know for sure it cannot be 0, since we live, and we can theoretically be detected by the electromagnetic waves we emit -- and far less likeley by artifacts such as the voyager 2 probe. So the likelyhood of a planet, capable of supporting life is not 0, but at least 1*10^-11 (100 billion stars exist in our galaxy, and at least one [the sun] has a habitable planet [the earth]).

That said, this particular parameter will likeley be higher, since we have found several exoplanets, that are estimated to be habitable.

it is generally assumed that N >= 1.

However it's worth pointing out that the Drake equation is couched in terms of probability so as long as N != 0 it's fine, there is no reason billions of civilizations can't exist no matter how asymptotically close to zero the probability of even one civilization is, it would just be weird.