Thanks for your reply.
But I do have only 3 users created so far, and each time when I run the code I could create one more user.
ie. the new user is created in every first iteration only at the time of the for-loop execution.
With the below code, I can create n number of users in TW.
services.CreateUser(userName, "Test Desc", null, "pwd");
Issue is only when I am using this code inside a for-loop.