Is Shin a first name or a last name?

Shin is both: a family name in some places and a given name in others. It is mostly a family name in South Korea; mostly a given name in Japan.

How to address someone called Shin

South Korea. Shin is a family name and is usually written first. Write Dear Mr. Shin or Dear Ms. Shin, or use the full name if you cannot tell which title fits.
Japan. Shin is part of the given name, which comes after the family name. Use it only on first-name terms; formally, the title goes with the family name.

Not sure which part of a full name is which? Check the full name with the country where the person is based.

People recorded in Wikidata who carry Shin as a family name or as a given name, by country of citizenship (860 in total).

Country	As family name	As given name	Family name share
South Korea	722	0	100%
Japan	2	91	2%
North Korea	14	0	100%
United States	10	2	83%
Canada	6	1	86%
Uzbekistan	3	0	100%

Wikidata covers notable people, so counts show how the name is used, not how many people carry it.

As a surname: about 21,735 people in the 2010 census, rank 1,656 among US surnames.

More on this naming tradition: How Korean names work.