Think about time. Anything between 5 and 10 clearly defined tasks in a 60–90 minute session is typical. You should test the core product functionality—login procedures, conversion funnels, and the like, before spreading your coverage throughout the product.
The way you talk about your tasks or questions is also crucial to getting useful results. Choose carefully between the direct tasks, the scenarios (Tingting Zhao explains the difference concisely here: https://design.canonical.com/2013/08/usability-testing-how-do-we-design-effective-tasks/), and the tasks, closed or open. Closed tasks have only one possible positive result, while an open task can produce some results and still be defined as successful. ...
