A new roof is a big job. And a big investment. That's why we strongly recommend that you hire a professional roofing contractor who can offer proven experience, a written estimate and worry-free installation. Check for references and call a few of them. Improper ventilation will damage your shingles and can invalidate your manufacturer's warranty.
Local codes determine the maximum number of nail-overs.
Additional waterproofing underlayment might be needed if ice damming, wind-driven rain, or collected or flowing water might be a problem. Four fasteners for each shingle - six in high wind areas. You should be very clear about who to call with a problem. Check warranty plus UL fire and wind resistance ratings. Choose a name you trust - not all shingles are the same.