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,

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.