So by now you should be able to figure out that the NACA 2021 doesn't make any sense. It would be a 2% camber located at 0% of the chord and 21% thick.

There's a few that really do make sense. For example the Helmut Quabek series of model glider airfoils had names that reflected their camber and thickness values. On the other hand Eppler, Selig and Martin Hepperle numbers appear to just be a series number and have nothing to do with the airfoil parameters.