The Productivity report on your HubSpot CRM Dashboard will display the total number of tasks to be completed in your selected time range, as shown here:
If this number does not reflect the expected number of completed tasks, check the timeframe that's set in filter at the top of your dashboard.
Tasks will only be reflected in the Productivity report within the timeframe of their set due date. So, for example, if you completed a task on June 1st that was due May 30th, the task would show in the month of May and not in the month of June.
For more information on using tasks in the CRM, check out this article.