Source: S.C. Code Ann. § 26-1-60. Seal of office; notary shall indicate date of expiration of commission. Each notary public shall have a seal of office, which shall be affixed to his instruments of publications and to his protestations. He shall indicate below his signature the date of expiration of his commission. But the absence of such seal or date prior to and after May 30, 1968 shall not render his acts invalid if his official title be affixed thereto.
I am assuming you are from South Carolina from your user name. Basically, what this means is a lack of seal does not invalidate a document, but it could cause the notary to be fined/liable for any financial damage from the lack of seal.
So, the answer to your question, is you must have a seal in South Carolina.