We did a retrospective on a previous deployment of our software at Zuzalu here: https://pse-team.notion.site/Zupass-Stamps-Retrospective-v1-6bd7aa223f4d41cdb59b4ecf8aafb130

In that document, we were able to compute the number of stamps collected and total number of people that interacted with the experience. Similar metrics will continue to be used for future grants.