FWIW, I have seen posts from people who scored 19 on the ACT secure scholarships. I am sure that whoever did that had some very impressive credentials in other areas. My point is this: prepare for the test as best as you can. Give it your best shot, and take it again in December if need be. But, at the end of the day, that test will not define you. Go ahead and make the best OVERALL application you can, have a great interview and let the chips fall where they may. As long as you can look yourself in the mirror and know you gave it your all, you can be proud.
My DS is in the process of completing his ROTC application now, and his ACT test score is probably the weakest piece of his puzzle so far. He's taking it again on Saturday and will go a third time if need be. Based on his credentials, I think he'd be a shoe-in for a scholarship with a solid ACT score, but I can't take the test for him or predict the future. One thing I do know: he is committed to seeing the process through, and he says if he doesn't get a scholarship, he's attending the college of choice, joining the battalion and competing like hell for a campus-based scholarship.
All you can do is all you can do.