Hello Sanjeeb,
I am not sure about the upper limit and what context you are referring to. A reader can have many dashboards/analysis opened in multiple tabs but reader can/will work(action) on only one tab at any given time and that will only count as active session. Other tabs may be there but inactive session unless some action is performed.
A Reader session starts with user-initiated action (for example, login, dashboard load, page refresh, drill-down, or filtering) and runs for the next 30 minutes. Keeping QuickSight open in a background browser window/tab does not result in active sessions until the Reader initiates action on page.
Hope this helps.
Cheers,
Deep