blog post limited or child resource limited

Hi, I want to limited blog post or child resource in modx. I created web site plan and limited resource for my customer . If customer buy small plan , it can create 20 post in blog news . It is possilbe ??

You may use Ditto to show only 20 documents with the &display=`20` parameter.

Thank you micro . But i want to limited when he saved the document ( ondocformsave - event ) , if it document parent have reached limited 20 , it show message ' Limited resource '. Ditto display limited only show in front end and not work in back end . My customers always created more documents .. .

Thank you for reply me and sorry for my silly english.