Do you know if this is her first clutch? If it is, then I would not expect 20 at all. Maybe 10-15. If you get too many eggs for you to handle, you can take the extras and put them in the freezer, then throw them away the next morning. Better to kill them in the egg than after they have hatched. Like my hatchling rack will only be big enough for 16 hatchlings, if I get 17 then 1 is going in the freezer.