How many nitro tabs before pt should call 911?

I am studying for my final in Med Surg II and am totally confused about nitro. I am doing practice NCLEX questions from an NCLEX book. The question was “Which comment by the client would indicate to the nurse that the client understands how to take his sublingual nitroglycerine?” The first answer was “I will call 911 if the pain does not go away within 5 minutes after taking the medication.” Wrong, I thought because I remembered in Pharmacology how my professor emphasized patients can take three tabs before calling 911. Anyways, my answer was wrong and the first answer was correct!