How to launch a Rule when X nodes were created in X days time?
I don't think rules by itself has the capability to do that - unless you get very deeply embedded into components, and by then it's almost the same as programming.

You probably need to write a custom module with a cron hook that checks how many nodes were created in the last 10 days. Do you want to know how many nodes were created at all, or just by a particular user? Either way that seems the best way to do it.