I have a 20 dollar bill of 1864 (confederate states America) with a number and signed. What is the value ?

Probably not that much. There are alot of these floating around relative to the people who want them. Its really going to depend on the condition. If its in really great condition it will be worth a lot. You need to take it to a collector, antique roadshow, or some sort of historical museum. They are going to be the only ones who can tell you.