What amount diphenhydramine is an overdose?

It depends: Diphenhydramine was a major cause of seizures reported to a poison center. At a dose of 7.5mg/Kg (a total dose of 500 mg [20 OTC tablets] for a 150 pound person) many p0eople will develop severe reactions. In doses over 1g (40 tablets) the drug may be lethal.

OD: “Lethal levels of diphenhydramine in the blood include higher than 19 mg/L, 7 mg/L and 1.5 mg/L in adults, children and infants respectively .” REF: https://overdoseinfo.com/diphenhydramine-overdose/