The 65th player is SourceTV and it’s a bugged slot. If you touch it in anyway (kick Torch) the server will shit itself and reset to 16 player max. The game only allows for 64 players.
obvioulsy if above 64 players would ever had been possible in this game somebody would probably already had figured it out over 10 years ago and not first now
Unfortunately custom servers are only for PUBG partners and even then are still hosted on their machines. Not sure if or when they will release the server files..