How to restrict number of nodes per content type?
Have you tried the Node Limit module?


  The Node Limit module allows administrators to restrict the number of nodes of a specific type that roles or users may create. For example, if a site has an "Advertiser" role that can create "advertisement" nodes, then the node limit administrator can restrict all users in that role to a specific number of nodes. He may also restrict users on a per-user basis.


You could create a role just for node limit and assign ALL users (including the admin) to that role.