The airfoil has to be thick enough to take a spar that is strong and stiff enough for the intended use of the model. This depends on what type of materials you could use for the spar. A strong stiff, light weight spar conflicts with the need to have a thin airfoil that penetrates well. Where the compromise falls, to resolve this conflict, depends on the purpose of the model, how it will be launched, the model's size and its wing loading.

The more you can tell us about this model, the better we can help you with harmonizing the airfoil selection with all the other aspects of the design.