You can keep 5 or 6 in a 20 long. ( but I would only keep as many as you can afford to feed) As far as them jumping out, probably not, but they will climb. I have clips on my screen cover just to be on the safe side. If you are going to have them in the basement, you will probably need a heater. I have never used a heater, but mine are in my livingroom and it stays about 74 degrees. They are fun to watch and will give you hours of enjoyment.