fewer than 1,000 people and more than 2 million people, so the rule of thumb
is that there is no rule of thumb).
If you are just starting, I would suggest segmenting your audience to around
10,000 people, because the higher level of filtration will allow your ROI to be
greater. Just keep in mind that it will take longer for you to reach your
campaign goals because the potential audience is a metric for 30 days of
advertising.
Congratulations, you have now located your audience on Facebook from
scratch!