The FCC announced Amateur Radio License applications now require a Federal Registration Number. A Social Security number will not be accepted. If you are licensed then you already have a FRN. It's on your license.
Do you need to have a license to apply for a FRN? No, Any US citizen can apply for a FRN. It's required to do business with the FCC.
An FRN is easy to get from the FCC.gov website:
https://apps.fcc.gov/coresWeb/publicHome.do