Limit number of child records to 10

Hi,

I saw this:

The best approach is to replace 'Add new' button with your own button (use 'Insert button' option). In this button server side code you can check the number of records already created and either redirect user to the Add page or display a message staying on the List page.

Now just need help with the code. How to check the number of records with a specific "Class Code" and only allow them to add if its less than 10.