"Rule 1.01 Baseball is a game between two teams of nine players each, under direction of a manager, played on an enclosed field in accordance with these rules, under jurisdiction of one or more umpires." The Official Baseball Rules are written for professional play. The chance of a pro team starting an inning with 10 (or only 8) players on the field is very unlikely. There are too many umpires, coaches, and managers for that to happen. But it would easily happen in amateur play that uses pro rules. The rules do not contain a penalty, but there is a pro interpretation regarding playing with less than 9 players. It states that all play that occurs with less than nine defensive players on the field is nullified and the inning (or portion of the inning) would have to be replayed. It does not address what happens if there are more than 9 players on the field, but I imagine the same would be true. The inning would have to be replayed. If a team starts a game with at least nine players but because of injuries, substitutions, and/or ejections can only field 8 eligible players, the team forfeits the game under pro rules. There are some amateur organization that allow a game to start with 9 players but the game is allowed to continue with less than 9 players.