Is it neo-nazis? Is it people who have the same racist and anti-semetic beliefs as neo-nazis, but try to put a more intellectual, respectable image on it? Paleoconservatives? Republicans who want less free trade and immigration? Young conservatives who say politically incorrect, outrageous things for shock value? Online trolls who make frog memes on twitter? Anyone right of center who deviates from mainstream, Reaganite conservatism? Anyone who supports Trump? All of the above? A lot of people who are considered to be alt-right would deny they're alt-right, and a lot of people who label themselves as alt-right would say others who are considered alt-right aren't really alt-right. The alt-right label has been placed on a lot of different things and people, but is there any kind of consensus on what it actually means?
You can say a lot of similar things about the Religious Right and to a certain extent, Neoconservatives. Except, this "movement" has literal Hitler supporters in it. My take is that these movements range in size, depending on how they are defined, from a handful of Elite PoliSci/Econ Professors, Wealthy Televangelists, and a handful of internet trolls to in the most broad sense, about 30-40% of the voters in a typical national Republican campaign. It means everything from "a decade amongst friends" to a populist campaign that appeals to a "basket of Deplorables" where traditional R campaigns, such as the ones in 90,92,96,06,08, and 12 have fallen short.
