Calling API using Parallel.ForEach not processing all records

Question

In below code driverList will have 100 unique drivers and based on the start date and end date, n number of driver activity will be generated for n number of drivers.

Here for example if driverList has 30 unique drivers and start date and end dates have a difference of two weeks then the total request that are sent to API is around 10K and it takes a lot of time to process. I would like to know if there is a way to process driverList parallely(i.e: Process multiple drivers to api at the same time).

I tried Parallel.ForEach but its not process all the requests, after some 50 requests i get no connection found.

Any lead is much appreaciated.

`foreach (var driver in driverList)
{
DateTime startDate = startTime;
DateTime endDate = endTime;
while (startDate.Date != endDate.Date)
{
    var driveTime = Constants.Constants.DrivingTimeInMinutes.OrderBy(i => Guid.NewGuid())
        .ToList();
    var restTime = Constants.Constants.DrivingTimeInMinutes.OrderBy(i => Guid.NewGuid())
        .ToList();

    Random rand = new Random();

    var activity = Constant.Activity[rand.Next(Constant.Activity.Length)];

    DriverActivity driverActivity = new DriverActivity()
    {
        DriverId = driver.DriverId,
        DriverName = driver.FirstName + " " + driver.LastName,
        Date = startDate.Date.ToString("yyyy-MM-dd"),
        Activity = activity,
    };

    switch (activity)
    {
        case "Driving":
            driverActivity.StartTime = startDate;
            driverActivity.EndTime =
                driverActivity.StartTime.AddMinutes(driveTime[rand.Next(driveTime.Count)]);
            break;
        case "REST":
            driverActivity.StartTime = startDate;
            driverActivity.EndTime =
                driverActivity.StartTime.AddMinutes(restTime[rand.Next(restTime.Count)]);
            break;
        default:
            driverActivity.StartTime = startDate;
            driverActivity.EndTime =
                driverActivity.StartTime.AddMinutes(
                    Constants.Constants.OtherTimeInMinutes[
                        rand.Next(Constants.Constants.OtherTimeInMinutes.Length)]);
            break;
    }

    startDate = driverActivity.EndTime;
    await ApiClient.UpdateDriverAcitivity(driverActivity);
    }
    Console.WriteLine($"Processed Driver : {driver.DriverId} successfully");
    }`