No, but most shirts sold in the US do.eightonecube said:all shirts have RN number??
To legally sell a shirt in the US (and most other places, but the laws are different for each country) it must contain a permanent label that has several pieces of information (such as care instructions, country of origin, etc.).
One of the things it must have is either the full name of the company or an RN (you could have both if you wanted, but there's no need). The full name would be something like Joe's Clothing, Footwear, & Textiles of Canada, PTY LTD - but that company may just be known as "Joe's Clothes" to its customers, and not want the full name to have to go on a tag. By registering for an RN the companies information can be publically known, without them having to fill a tag just with their name. It's all about being accountable.
So while not all shirts sold in the US are required to have an RN, the practical reality is that most legal shirts do.