So, you need to ask yourself a few questions:
- How is a unique user identified? Is it just the serialNo? Is it a combination of fields?
If it's a single field, you can count unique values fairly simply if it's a combination of fields, you'll need to do a scripted query as described here:
Once you have a query that gives you a count of the distinct values, you can display an aggregate. Here's a question and answer about how to display a percentage.
One last note: it may be easier to massage your data so that it's stored in a way that makes the Kibana queries easier to write and reason about.