Incident is an individual occurrence; incidence is the rate of occurrence. We seem to get into trouble in the plural. ...There was an incident last night that required an ambulance. ...There were several incidents during the night. ...The incidence of violence crime in the area has decreased. Incidences is rarely used. It would have to mean the "rates" of something that happens. And "The incidences are too numerous to name" is just plain wrong. Happy punctuating! Margie