What is the reason for such a restriction.
As far as I know there is no lowlevel option (like a jcr mixin-Type) to restrict nodecreation. One of the basic problems is, that a user (and also the API calls executed for his principal) might not see all existing pages/nodes based on his ACLs. Internally there are mechanisms to optimize large amount of nodes (afaik, the usage of a b-tree was introduced, so internally the tree is broken down in subbranches). If your use-case is from the high-level perspecitve (like restrikting the max-amount of navigation nodes) you might have the need to overlay the ExtJS frontendfunctionallity of CQ to apply some prechecks before a user can perform a node-creation. But you should be aware that such solutions can in most cases be tricked (like "moving" pages).
We are storing social network content e.g. tweets into nodes & it could grow exponentially, so we would like to restrict the amount of soical content that can be stored in CQ. That is the reason I would like to restrict the node creating to certain number so that queries to that content will not pose threat to performance.