The minimum will depend completely on the technology and the economics
If we are crawling along at 2% c, we will have enough problems in just building a ship that will have a reasonable chance of survival.
If we make breakthroughs in bioscience and can safely freeze people, or we can do parthogenesis or cloning, we could do different things.
If we make the breakthrough into the holy grail of warp, we can maybe send as many or as few we want.
Ed