RE:[PHP] sessions...how to use not clear?

What are the chances of two people getting the same session number in a thousends of day operation? When you down the size of that to a couple hundeed a day... WHy do we need to have a session name for that? because eventually i want to take the user name (session name/variables/user info) and create a preferences section of the website im doing and that would require a constant session name from visit to visit