How rare should planets be when rare habitable is chosen?

I am well into mid-game on a huge map. At least half the galaxy is visible and all of the races have only their home planets.

I made the choice just to see how it would go and to cut down on planet management, but I figured on a huge map there would at least be a couple of dozen out there up for grabs. Is there a formula for each of the choices to give a range of what should be expected?