First rule of EMS is as follows. Patients don't learn numbers or rules that your textbook teaches you.
No one, and I mean no one, should be trying to use a BVM on a patient with a RR of 12 or 28 as was mentioned earlier in this thread. Respiratory rates vary much, much more than the "12-20" for adults taught in EMT school. 8 can be plenty with adequate tidal volume in some people at rest, and 30 is simply a minor asthma attack or anxiety most of the time.
Hypoventilating patients in need of assistance with a BVM will be abundantly obvious most of the time. Cardiac arrests with agonal or absent respiration are obvious times to use a BVM, as is the hypoxic opiate OD that will typically present with RR between 0-4ish.
Conscious, hyperventilating patients are rarely going to allow you to use a BVM on them. If you're a BLS unit with a conscious patient in severe respiratory distress and a dramatically elevated RR, a non-rebreather (better yet CPAP if local protocols allow) will be the best choice the vast majority of the time. I've run a lot of critical respiratory patients, and I've yet to use a BVM on someone who is conscious and tachypneic. Experiences may vary from other posters.
If you're trying to learn the various indications for the written test, don't overthink it. NREMT loves to overemphasize NRB usage, but keep in mind most patients don't need one at all. Mildly hypoxic patients with minor respiratory distress are often fine with a nasal cannula, moderate to severe distress consider NRB at the BLS level, and for apneic and severely hypoventilating unconscious/semi conscious patients, use a BVM. If a test question wants you to choose BVM on a conscious and hyperventilating patient (again, extremely rare in real life), it will likely describe the respirations as extremely rapid, inadequate, and shallow. Follow these cue words and you should be fine. Good luck, and I recommend asking your instructor for further clarification.