Reg E requires us to inform the customer of the results of our investigation. Is there anything to prevent us from notifying them of numerous disputes using just one letter? If the letter clearly includes the required information for each disputed item that the customer made, do you see any reason why this would be a problem. Does anyone else send a single letter for multiple disputes?

I don't see an issue with it. A written response to the results of an investigation is only technically required when you are denying their claim or asserting that there was a difference.