int                 threadsCount;
static readonly object syncLock = new object();

        public static void Wait(double seconds)
        {
            int ticks = System.Environment.TickCount + (int)Math.Round(seconds * 1000.0);
            while (System.Environment.TickCount < ticks)
            {
                Application.DoEvents();
            }
        }

        private static void FilterGetRatings()
        {
threadsCount = 0;
int MaxPage = 100;
int MaxDegree = 20;

                for (int j = 1; j <= MaxPage; j++)
                {
                    threadsCount++;
                    Thread ths = new Thread(() => FilterGetRating(j));
                    ths.Start();

// Жду когда освободится поток                    
                    while (threadsCount >= MaxDegree)
                    {
                        Wait(1.0);
                    }
                     
                }
// Жду пока все потоки доработают
                while (threadsCount > 0)
                {
                    Wait(1.0);
                }
}

private static void FilterGetRating(int i)
        {
            //что-то делается

            lock (syncLock)
            {
                threadsCount--;
            }
        }

    class Program
    {
        static int Tasks;

        static void MyFunc(int i)
        {
            System.Threading.Interlocked.Increment(ref Tasks);
            System.Threading.Thread.Sleep(2000);
            Console.WriteLine($"ThreadId: {System.Threading.Thread.CurrentThread.ManagedThreadId},\tparallelism: {Tasks},\tvalue: {i}");
            System.Threading.Interlocked.Decrement(ref Tasks);
        }

        static void Main(string[] args)
        {
            Parallel.For(0, 1000, new ParallelOptions { MaxDegreeOfParallelism = 20 }, (i) => MyFunc(i));
            Console.WriteLine("Done!");
            Console.ReadKey();
        }
    }