One set should be fine for 15s but it doesn't mean that you are limited to doing 15 reps with the initial loads. You might want to do some more reps to get a really good burn or just slow down the tempo to get a similar effect.
Try to think in terms of total reps for a cycle. If you start off with 15 reps you might want to try to keep that constant throughout the cycle. Or, after a bunch of cycles, 20 reps might be a good total to shoot for. It depends on what's working for you. There is no sense in doing more work than you need to to trigger a growth response. The tricky bit is that no-one can say for sure how much that is for an individual. If 15 reps worked for you the first cycle just stick with that and up your RM target loads.