The plurals of last names are just like the plurals of most nouns. They typically get formed by adding -s. Except, that is, if the name already ends in s or z. Then the plural is formed by adding -es.

Is it the Smiths or the Smith's?

The plural of Smith is Smiths. NOT Smith's. And if for some reason the Smiths wanted to use the possessive, they would have to use the plural possessive. ... So, the plural possessive would be the Smiths'.

Is an apostrophe needed to pluralize a last name?

Don't use an apostrophe to make your last name plural. Apostrophes can be used to show possession—à la the Smiths' house or Tim Johnson's pad— but they don't indicate there's more than one person in your family.