Program.cs

using System;
using System.Net;
using System.Net.Sockets;
using System.Threading;

class Program
{
    static void Main()
    {
        for (int i = 0; i < 1000; i++)
        {
            new Thread(() =>
            {
                var listener = new TcpListener(IPAddress.Loopback, 0); // <s>RTFM</s> MSDN 
                listener.Start();
                var port = ((IPEndPoint)listener.LocalEndpoint).Port;
                Console.WriteLine(port);
            }).Start();
        }
    }
}