If your users are submitting content with CRED forms then you would have a couple of alternatives, each of which requires writing some custom code.
The first would be to not display the form to submit posts at all if the user had already published the maximum number of posts.
For this you would need to register a custom function or shortcode that ran a query to get the posts of that user and return the count. You could then use that in a wpv-conditional shortcode (after registering the shortcode or function at Toolset > Settings > Front-end Content) which you use to either reveal or hide the form to publish content.
The second option would be to allow users to see the form and try to submit content, but use custom validation to reject the form if the user had already submitted the maximum number of posts (which would again require a custom query to get the posts of that user), using the cred_validate_form hook: https://toolset.com/documentation/programmer-reference/cred-api/#cred_form_validate
If you prefer one or the other and need help, let me know.
