example_semaphore.cs

class Program
    {
        static void Main(string[] args)
        {
            var timeExecution = new Stopwatch();
            Console.WriteLine($"Iniciando a execução");

            timeExecution.Start();
            InitProcess();
            timeExecution.Stop();

            Console.WriteLine($"Execução finalizada: {timeExecution.Elapsed:m\\ss\\.fff}");
            Console.ReadKey();
        }

        static void InitProcess()
        {
            var semaphore = new SemaphoreSlim(1);
            var listTasks = new List<Task>();

            for (int i = 0; i < 20; i++)
            {
                listTasks.Add(ProcessWithSemaphore(semaphore));
                //listTasks.Add(ProcessSemaphoreless());
            }

            Task.WaitAll(listTasks.ToArray());
        }
                
        static async Task ProcessWithSemaphore(SemaphoreSlim semaphoreSlim)
        {
            await semaphoreSlim.WaitAsync();

            Console.WriteLine("Simulando a chamada a uma API qualquer com semaphore");

            var _httpClient = HttpClientFactory.Create();
            await _httpClient.GetAsync("http://httpstat.us/200?sleep=1000");

            semaphoreSlim.Release();
        }

        static async Task ProcessSemaphoreless()
        {
            Console.WriteLine("Simulando a chamada a uma API qualquer sem semaphore");
            var _httpClient = HttpClientFactory.Create();
            await _httpClient.GetAsync("http://httpstat.us/200?sleep=1000");
        }
    }