According to my lawyer the tax number needs to be apostilled. This was the instruction my lawyer gave me - exact words:
Certified copy of the tax number of the country of residence (or origin), translated into Portuguese, apostilled or double certified (stamped by the Ministry of Foreign Affairs and afterwards stamped by the Portuguese Competent Embassy or Consulate); valid for 1 year once issued.
In the UK you can download a ‘National Insurance letter’ from your online tax account. This letter states your NI number. A notary will notarise this which you then send off for apostille (legalisation in the UK). Then you send this to PT. Regarding the translation into Portuguese seems as though my lawyer has sorted this out since I haven’t. But perhaps it is different for US applicants/US Social Security cards, or perhaps some SEF offices/clerks aren’t too bothered whether it’s been apostilled or not. If they told you it doesn’t need to be apostilled then you’re probably fine.