How do I limit the node count of a content type to one per user? I have a custom content type. I would like to limit that each user can only create one node of this content type. How can I achieve this?
You can use the Node Limit module for that:


  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.