Hi Coline,
There are a couple of ways in which you could do this. The simplest in the case of looking at one or two recipients would be to simply navigate to the delivery in question and take a look at the delivery and tracking logs, filtering by recipient. That should give you the number of times the Open was triggered.
If you need to do this for a larger group, however, you would need to create a workflow that queries the recipient tracking log table and filter based on delivery and URL type. That should give you the info that you are looking for.
Regards,
Craig