How to find number of assigned content to all user
Basically you need to add the entity reference field using relationship to the view. Then use aggregation to find the count.

Refer https://www.ostraining.com/blog/drupal/entity-reference-views/ on how to create view using entity reference field. 
To find the count, you can use aggregation. For using aggregation you can refer https://drupal.stackexchange.com/a/31542/48290.