Depending on where you put it, you might need to use a while function.

end_at_round(round)
{
	while(1)
	{
		if(level.round_number == round)
		{
			level notify( "end_game" );
			break;
		}
		wait(.05);
	}
}

At the top of the script, under main(), put:

level thread end_at_round(2);

Make sure to change the round from the function call to the round number you want the game to end at