using System;

using System.Linq;

using System.Collections.Generic;

using System.Threading.Tasks;

using System.Threading;

​

public class Program

{

    public static void Main()

    {

        var dealIds = new List<int>();

         for(var i=0; i < 100; i++)

         {

             dealIds.Add(i);

         }

         int lpCount = 0;

         List<Task> tasks = new List<Task>();

        foreach(var dealId in dealIds){

             lpCount++;

              tasks.Add(Task.Run( () => {

                   Thread.Sleep(1000);

                    Console.WriteLine("WELCOME : " + dealId);

              }));

             if(lpCount == 20){

                 Task.WaitAll( tasks.ToArray() );

                  tasks = new List<Task>();

                 lpCount=0;

                 Console.WriteLine("***********************************************************");

             }

         }

    }

}

​