No, there is no explicit limit of reserved servers per experience.
No, but kind of. Reserved servers operate in the same way as public servers. They will automatically close a short time after all players in the server disconnect. To “manually” shut down a reserved server, you could kick all connected players.
Unclear question. Reserved servers cannot be joined in the same way as public or private servers. Roblox automatically attempts to place players in public servers by prioritizing servers that the joining player has friends in among other things. Private servers are essentially just like public servers except the owner of the private server can decide which players (usually the private server’s owner’s friends) can join. In order to teleport a player to a reserved server, you must either call the method TeleportService:ReserveServer() or have access to the ReservedServerAccessCode by some other means. The reserved server will be created when the access code is first used.
Access codes remain valid indefinitely, meaning reserved servers can still be joined if no game server is running (in this case a new server will be started).