I have read that moss can damage or shorten the life of an ashpahlt shingle.what if its just small patches of moss here and there on a roof will that hurt anything?

The problem is that it can build up on the edges of the shingle and raise it. Then water can get underneath. If you search " removing moss from a roof" you will find suggestions. You may want to trim back tree branches to allow more sun on the roof. If you can get copper or zinc sulfate, a dilute solution of the crystals in water, sprayed on the moss, will kill it.