Hi! Have you looked at the pre-built "Usage - Dashboards" dashboard provided by Sisense? This has a few charts that already show dashboard viewers. In short, to get the views of a dashboard by user, you would need that information from the "FactActions" table in the Usage Analytics Model. DUPCOUNT does work on non-numeric fields. So if I wanted to see the number of views a dashboard got per user, I might bring in the following information: username from DimUsers, dashboardID from FactActions, and DUPCOUNT(username). This would show me the views per dashboard by user.
Reasons you may get an "unrealistic" number is that the FactActions table is capturing a lot of actions. So if I am viewing a dashboard, I might select different filters which is causing the dashboard to reload. All of these events might be viewed as me viewing the dashboard, but in reality, I was just viewing it once and interacting with it. Play around with filters from FactActions to see if there are any ActionTypes that you can filter out to get the number down. Hope this helps!