max_length attributes works only for Single-line text, Wide single-line text type of variables.

You can write onsubmit client scipt for your catalog item to restrict length less than 9 digits like below using regex

Thanks to @Mark Roethof  for below article

https://community.servicenow.com/community?id=community_article&sys_id=78785137dbcbf3801cd8a345ca961...

Regards,

Sachin