Don't use an apostrophe to form the plural of a family name. Clay and DeeDee Smith are the Smiths, not the Smith's. Don't use an apostrophe to form the plural of a family name even if it ends in a vowel.

Smith is most famous for his 1776 book, The Wealth of Nations. Smith's writings were studied by 20th-century philosophers, writers, and economists. Smith's ideas—the importance of free markets, assembly-line production methods, and gross domestic product (GDP)—formed the basis for theories of classical economics.

As towns grew, the demand for Blacksmiths increased so it is no surprise that numerous families were adopting Smith as their surname. A Blacksmith was a consistently important profession for centuries and was linked to the survival of a town. Other last names were less common as other professions were less significant.

It dates back to the Anglo-Saxon era and derives from the occupation of 'smith', from the Old English 'Smid', which means to hit or to strike, and was used to describe someone who worked with metal (a blacksmith for example).