When the NOAB sits, they will assess all candidates, and group them into 3 categories. Acceptable with offer, acceptable without offer, and unacceptable. (Note, the actual category name may be different, but that's the idea). If there are 25 candidates who want to be Mars Bars some day, and 14 slots available at that time, and 5 of the candidates are deemed to be unacceptable, 6 of the other candidates will be told, hey, you'd be just fine, but we can't quite fit you in at this moment.
It is important to note that those 6 candidates may indeed still be getting an offer if any of the 14 given an offer are unable to start basic training, either due to injury, family emergency, or just plain old deciding they don't want to anymore. On my basic training course, we had at least two Naval Technical Officers whom this happened to.
One thing I'm not sure about is whether or not a candidate who is acceptable but didn't get an offer will be required to attend subsequent NOABs in order to remain in consideration, or if the next NOAB will simply use the rating from the previous board in the next round of assessments, should the person still want to get in at that point.