Using thread.join to make sure all threads in a collection execute before moving on

Is there a problem with this type of implementation to wait for a batch of threads to complete before moving on, given the following circumstances?:

• CCR or PFX cannot be used.
• Customer.Prices collection and newCustomer are NOT being mutated.
• CloneCustomerPrices performs a deep copy on each of the prices in Customer.Prices collection into a new price Collection.

public List[Customer] ProcessCustomersPrices(List [Customer] Customers)
{
[Code to check Customers and  deep copy Cust data into newCustomers]
List[Thread] ThreadList = new List[Thread]();
foreach(Customer cust in Customers)
{
ThreadList.Add(new Thread(() => CloneCustomerPrices(cust.Prices, newCustomer)));
}

Action runThreadBatch = () =>
      {
           ThreadList.ForEach(t => t.Start());
              ThreadList.All    (t => t.Join([TimeOutNumber]));
      };

runThreadBatch(CopyPriceModelsCallback, null);

[More Processing]
return newCustomers;
}

From stackoverflow

• Makes sense to me, so long as the threads finish by the timeout. Not sure what newCustomer is (same as cust?). If that's the case I also don't know how to plan to return just one of them.

• The waiting implementation seems fine, just be sure that CloneCustomerPrices is thread safe.