Accurate legal guidelines

Seems to be a hypothetical. There is no necessity to mention Aadhaar card number either in a government/a private organisation. 

Will it be illegal to display the number or masked number is okay? 

In most cases, when you share your Aadhaar, you'll be using the masked Aadhaar version, which displays only the last four digits of your Aadhaar number. 

This is done to protect your privacy and prevent potential misuse of your full Aadhaar number. The remaining eight digits are replaced with asterisks or "xxxx". 

Sharing your full Aadhaar number can expose your sensitive information to potential fraud and misuse. 

The masked version minimizes this risk by only displaying the last four digits.

I endorse opinion and advise of learned senior experts. Use masked Aadhar number if it is a real story and not an examination hall question paper.

Aadhar full number can't be mentioned in ID card of any company. 

Yes sir and could you please share the section of Aadhar and any other guidelines which clearly mentioned this to avoid displaying full number?

It is appropriate and better to seek professional services of a local prudent lawyer for analyses of facts, proper guidance and necessary proceeding, if it is your personal and real problem.