The real problem is that if the number is greater than 1, what happens when one of those gets interstellar? Assuming the massive understanding to build interstellar travel didn’t evolve in a progress vacuum, genetic mastery can be assumed pretty safely, hence more than one likely becomes millions over time. It may start with workforce help or intelligent companion animals but it will happen. Or discounting that, think of all the worlds that are like earth was before life showed up? I guarantee an advanced civilization would import organisms to kick start the process and over the timescales of such a project one or two worlds could easily be forgotten about giving the chance for a new intelligent being to come about.