As a Roblox developer, it is currently impossible to teleport more than 50 players in a single party using the provided TeleportPartyAsync API.
If Roblox were to implement this it would allow us to create games where we teleport large quantity of players into a single server, for example open world games or Royale games, etc.
I think this is a much needed update seeing as we can make 200 player servers, so updating the API would let us make use of that space provided 
You should probably be using reserved servers if you want to move so many people. In Battle Royale games, you would always use Reserved Servers since you don’t want people joining in after the match starts.
The only case you’d need TeleportPartyAsync is if you’re trying to teleport a group of players into a server and want other players to be able to freely join it from the website through normal matchmaking. You’re going to provide more information about what you need this for in order for it to be considered as a feature request.