Your on-sight level is the highest grade you can climb first try, without falls. To be considered a true on-sight, you must do the climb without any prior coaching. You only get one try to on-sight or flash a route. If the route requires more than one attempt, you will be going for the redpoint, which means doing the climb from bottom to top without falls. Of course, the fewer attempts you need the better, but whether you clip the anchors on your fifth try or your 50th, you have redpointed the route.
Most people can redpoint nearly a full number grade higher then they can on-sight.
So, if you can consistently climb 5.11c first try, then a 5.12b should be feasible after a few days practicing and attempting the route. One 5.12b on-sight does not mean that a 5.13 redpoint is just around the corner. The following will teach you the key danger zones of a one-pitch sport climb and how to handle them. Sport climbers take frequent and forceful falls. Most serious sport-climbing accidents stem from mistakes made before the action even begins.