How do I run maximum no. of parallel threads for maximum output

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C#

C#4.0

threads

TPL

I have written an application to see how we can use tasks.
I am starting 50000 tasks in one go with minimum thread pool set to 32500 ( which is the limit for a 64 bit application) and inserting a record into db in the function with a sleep of 5 sec.
If I set the min. thread pool to 8000 threads it is writing 15-16k records/minute.
But on increasing the no. of threads it reduces to 2-3k.
I am not able to pinpoint for the reason for this reduce.
i have built the app in vs 2012 .net 4.5.
What can be the reason for this?
Whether there is some other limit in .net or windows or some hardware related issue.

static void Main(string[] args)
       {
           int minThread = 32500;
           ThreadPool.SetMaxThreads(Int32.MaxValue, Int32.MaxValue);
           ThreadPool.SetMaxThreads(minThread, minThread);
           Start();
       }

       static void Start()
       {
           int count =50000 ;
           Task[] taskArray = new Task[count];
           for (int i = 0; i < taskArray.Length; i++)
           {
               Task task1 = null;
               task1 = Task.Factory.StartNew(() => function());
               taskArray[i] = task1;
           }
           Task.WaitAll(taskArray);
       }

       static void function()
       {
           do
           {
               Thread.Sleep(5000);
               using (TestEntities db = new TestEntities())
               {
                   Reponse res = new Reponse { datetime = DateTime.Now, message = Thread.CurrentThread.ManagedThreadId.ToString() };
                   db.Reponses.Add(res);
                   db.SaveChanges();
               }
           }
           while (true);
       }