Generally, the end of the bunk/roller should be within a few inches of the transom (meaning, they should be "short" of the transom). If it's rollers, they obviously CAN NOT be behind the transom. But if you have bunks, the bunks could actually stick out past the stern... as long as you don't have trim tabs or transducers. It can still be done, but it's generally safer to not.
There is no hard and fast rule to the length of the bunks or the number of rollers. But more is never a bad thing. (1) set of bunks, about 8' or 10', is enough for that boat. But again, more won't hurt.
Do the measurement process I mentioned above and compare that to the trailer - from bow roller/winch stand to then end of the bunk/roller. Don't forget weight capacity. Take your dry weight, add engine/options/batteries/fuel/etc.
EDIT: Lee, you could also research the make/model of the trailer you're looking at. Either through the company's website or, usually the better way, by contacting them directly.