Section 203. Player eligibility (a) Teams entering the Open Cup shall use their official league roster as their Open Cup roster. Except as specified in this Policy or except as the result of discipline matters imposed by U.S. Soccer, all players on an official league roster will be eligible for Open Cup competition, regardless of any status (e.g. injured reserve) each player may have with regard to league competition. Players on loan from another team or league may not be included on an Open Cup roster, with the exception of the following: 1) Loaned players originating from teams competing in leagues that are not active participants in the Open Cup Tournament or qualifying process (professional indoor leagues, foreign clubs, etc.); 2) players on loan from other US clubs for a period of greater than 90 days; or 3) players from teams who have been deemed ineligible for the current Open Cup competition due to the provisions of Section 202(d) and are loaned to the team exercising roster control pursuant to Section 202(d). A team may list up to 18 players on its game day roster. Professional teams may have no more than 5 foreign players listed . Amateur teams are not restricted as to the number of foreign players they may list. Foreign players shall be those players who are not protected individuals as defined in 8 U.S.C. § 1324b (e.g. U.S. Citizens, lawful permanent residents, asylees and refugees). Any conflicting Outdoor Professional League roster rules shall not apply to the number of foreign players allowed to compete in the Open Cup.
This is what I found in the US open Cup handbook from 2019.