For Sitefinity 5.1+ you can use the free admin Dashboard from Falafel.
It shows a number of stats like Content stats, media stats, etc.
You can find more information here or watch a video here.
Alternatively, you can check the attached webform. It has the code that counts the content items. From there feel free to create a custom widget that shows the number of the content items and place it in the dashboard.
Hope this helps.