Toilet over flowed upstairws now water downstairs, howmuch damage could have been done

You're not telling us much, so how would we know?
Was the toilet full of urine and or feces? What is the floor made of? What is the ceiling below made of? What kind of room is below?
Get out the bleach and start mopping and bring in some large fans and leave them on for 2 or 3 days, is about all I can suggest from what you have said.
Most of us have had to do this at least once in our lives.

Hard to say without removing a part of the ceiling. Usually there is no insulation between floors and for the most part, fecal material will probably not get through-e.coli maybe! Get things dry as possible, open a small hole under the leak and spray a light bleach solution in the ceiling cavity. Light, not drenching. Let it dry out and patch the hole. Should be good to go. LOL.