How to repair hole that the chair leg screws into
Repair it using the same hardware it originally had. It sounds like the chair had a T-Nut inserted:


Assuming the "male" screw in the leg still has good threads (you did not indicate that it was bad), buy a matching T-Nut at a good hardware store.

Note- there are quite a few sizes and types of T-Nuts, bring a sample from one of the other legs/chairs.