Determine the instance number for Godot run multiple instances feature

startup.gd

var _instance_num := -1
var _instance_socket: TCPServer 

func _init() -> void:
	if OS.is_debug_build():
		_instance_socket = TCPServer.new()
		for n in range(0,4):
			if _instance_socket.listen(5000 + n) == OK:
				_instance_num = n
				break

		assert(_instance_num >= 0, "Unable to determine instance number. Seems like all TCP ports are in use")	
			
		print("We are instance number ", _instance_num)