What is National ID number?

Some countries have national ID numbers. It's generally a number used on a national ID card. If your country doesn't have one, leave it blank. For Americans, we generally use our Social Security Number as a default national ID number. (Although there is some argument over the safety and lawfulness of requiring that number for private use, as some institutions do.)

Hong Kong China

the national ID number is the digits number writing on the card your government gives you to identify you as either a resident or citizen of that particular country...