3 as Alex already wrote. Dortmund II (one player over 23) and Stuttgart II (3 players over 23) are both confortable with their roosters. I checked also the team roosters of some Regionalliga reserves and detected no club with more than 3 over 23 players in the complete rooster, HSV II has not even a single player older than 23, if the transfermarkt.de roosters can be trusted. So the restriction does not seem to be a problem in order to establish a reserve team in lower 3. Liga, top Regionalliga level.