Well lets make numbers up. Lets imagine Capella has some 250 million people and that the majority of systems will be somewhere between a hundred million and a billion, totaling like 5 billion.
Let's imagine people spawning somewhat like post-WW2 but x2 or x3... let's imagine these people all having children, 5 per couple. To reach 5 billion in a single generation, you'd need to start with 1.4 billion people. If we say it's more like 1.5 generations, then it becomes something like almost a billion people. Those people must have come from Sol directly.
Alternatively, we can imagine a giant birthing policy. In vitro stuff, massive school systems, "parents" having each (couple) 30 kids. In this scenario, you'd "only" need ~300 million people (one generation), 60-100 million if we calculate with "more" than a generation.
But this program is unheard of, it's sheer speculation.