c# entity framework with threading

I have to run a once off C# calculation on millions of rows of data and save the results in another table. I haven't worked with threading in C# in a couple of years. I'm using .NET v4.5 and EF v5.

The original code is something along the lines of:

public static void Main()
{
    Stopwatch sw = new Stopwatch();
    sw.Start();
    Entities db = new Entities();
    DoCalc(db.Clients.ToList());
    sw.Stop();
    Console.WriteLine(sw.Elapsed);
}

private static void DoCalc(List<Client> clients)
{
Entities db = new Entities();    
    foreach(var c in clients)
    {
       var transactions = db.GetTransactions(c);
       var result = calulate(transactions); //the actual calc
       db.Results.Add(result);
       db.SaveChanges();
    }    
}

Here is my attempt at multi-threading:

private static int numberOfThreads = 15;

public static void Main()
{
    Stopwatch sw = new Stopwatch();
    sw.Start();
    Entities db = new Entities();

    var splitUpClients = SplitUpClients(db.Clients());

    Task[] allTasks = new Task[numberOfThreads];

    for (int i = 0; i < numberOfThreads; i++)
    {               
        Task task = Task.Factory.StartNew(() => DoCalc(splitupClients[i]));
        allTasks[i] = task;             
     }  

    Task.WaitAll(allTasks);             
    sw.Stop();
    Console.WriteLine(sw.Elapsed);
}

private static void DoCalc(List<Client> clients)
{
Entities db = new Entities();    
    foreach(var c in clients)
    {
       var transactions = db.GetTransactions(c);
       var result = calulate(transactions);
       db.Results.Add(result);
       db.SaveChanges();
    }    
}

//splits the list of clients into n subgroups
private static List<List<Client>> SplitUpClients(List<Client> clients)
{
    int maxPerGroup = (int)Math.Ceiling((double)clients.Count() / numberOfThreads);

    return ts.Select((s, i) => new { Str = s, Index = i }).
                        GroupBy(o => o.Index / maxPerGroup, o => o.Str).
                        Select(coll => coll.ToList()).
                        ToList();           
}

My question is:

Is this the safe and correct way to do it and are there any obvious shortcomings (especially with regard to EF)?

Also, how do I find the optimum number of threads? Is it the more the merrier?