When you get 16 credits you get a R/T ticket that is really two one way tickets. You can use it for a R/T, two people one way or even for two different one way flights. As an example if you have one R/T ticket but two passengers it's often better use your ticket for both passengers for the more expensive leg of your trip, particularly if you can get a DING fare for one of your legs.
As was previously posted you need 16 credits, SW doesn't give you one ticket at 8 credits.