From wikipedia on Psilocybe cubensis: "The concentrations of psilocin and psilocybin, as determined by high-performance liquid chromatography, are in the range

From wikipedia on Psilocybe cubensis: "The concentrations of psilocin and psilocybin, as determined by high-performance liquid chromatography, are in the range of 0.14–0.42% and 0.37–1.30% (dry weight) in the whole mushroom" My napkin math: 20mg is equivalent to roughly between 1.1g to 3.9g of dry weight (lots of variance in potency). This weight range is roughly equivalent to typical recreational dosages.