What was the most lashes a guilty man could receive under Deuteronomy 25?

Answer: Forty

A guilty man could receive no more than forty lashes, lest he be beaten more and be degraded in his brother's sight.

“He may receive no more than forty lashes, lest your brother be beaten any more than that and be degraded in your sight.”— Deuteronomy 25:3 (BSB)