As the question says - How to find all activities generated by one schedule reminder ?
You can find all activities of type 'scheduled reminder'. This shows a few fields, but not which instance of the scheduled reminder created this activity. You can filter the search by activity 'subject' and 'details', but I don't think the instance of the scheduled reminder is in there (?).
Using the api (explorer) you can get a lot of relationships to activities, like 'medium' and 'parent', but I'm not sure what these mean. Is that information (which scheduled reminder created this activity) lost ?