Depending on your location and the type of shingle you are using (3-tab, architectural, etc.), code determines how many nails per shingle must be used. Even though not required in all instances where I am - due to anticipated winds, I nearly always use five(5) per shingle, but in areas where high wind exposure could be an issue, I go with six. The cost of nails versus the cost to repair/replace an insufficiently fastened roof is minimal.
Here's a good link for IBC (International Building Code) information, but I would still suggest checking your local code, as states like FL, for example, require six. The reasons for doing this is for both peace of mind and whether or not - in the event of a claim, your insurance will cover damages.