If my math is right, each team had roughly an entire other team of players on the bench (10-11 players) and since this is a U-20 team, there's a good chance they still have some fieldable players in their training pool. Mamore declared they don't have enough left to field the minimum 11 for their next match though. As for red cards to board members, it's not entirely out of the ruleset, as coaches can and have been expelled.

How did they still have a team after 43 players got reds?

They didn't - the match was abandoned. And they didn't even bother with the second leg of the tie, the team that was 3-1 down just agreed to forfeit 😁