A RN number is a Registration Number. It’s required by law to be placed on your care labels. It’s there so your customers can identify you if something happens. (Worst case scenario is they have an allergic reaction to the fabric and want to sue you; best case scenario is a button falls off and your customer wants to contact you for a replacement.) A lot of new designers have absolutely no idea that this is needed, so don’t freak out if you don’t have one and have already been selling! Hopefully you included your company name on the care label, so there would be some sort of way to identify you. If you glance at the care label on the shirt or garment you happen to be wearing at the moment, you will notice a tiny RN # somewhere on there.