Hey

In the right corner of the serial number column in the CAT application form you will see a question mark symbol. You need to click on that symbol that will take you to the official website of National Backward Class mentioned below,

http://ncbc.nic.in/User_Panel/CentralListStateView.aspx

Select your State and search you caste in the list and there you will get your serial number.

Hope that helps .

Thank you.