Here's a care sheet on them:

http://www.lllreptile.com/info/libra...eror-scorpion/

It doesn't mention how many you can have in a 20gal and I have no idea (I only have one), so hopefully someone else can chime in or a google search can help you out :]

Also, don't be intimidated by the high humidity. As long as their substrate is moist, it's probably fine.