The record is five. Hahnemann, Keller, Dempsey, McBride, and Bocanegra.Arronson, Adams and Pulisic all on the field at the same time. Is this the most Americans playing in an individual PL game?
https://int.soccerway.com/matches/2008/04/12/england/premier-league/reading-fc/fulham-football-club/458835/