How to count the node a user has created in a View?
you have to use the Aggregation setting and active it in your view.


Activate Aggregation. (It's in the "advanced settings" of the view)
As your view is a view of users, add the relationship "User : content authored". If you set the option "Require this relationship", you'll skip users with no nodes created (can be useful)
Add your user name field, don't use aggregate
Add a node field, let's say Node : nid with the relationship set
Use Aggregate for this field, "Count DISTINCT".


Have fun !