Brazilian is the adjective built on the country name Brazil. It goes in front of a noun (Brazilian coffee, a Brazilian player) or after be (She is Brazilian). English always writes it with a capital B, even in the middle of a sentence, because it comes from a country name. It does not change for number, so several people are all Brazilian, not Brazilians, when the word is used as an adjective. Note that Brazil is the country and Brazilian is the describing word — English keeps those two apart, and one cannot stand in for the other.

This is the same word used as a noun for the person. It is countable, so it takes a and a plural: a Brazilian, two Brazilians. The capital B stays. English builds this pair for many countries — Italy gives an Italian, Canada gives a Canadian — and Brazil works the same way. Saying She is Brazilian and She is a Brazilian both work, though the first, with no a, is the more usual way to tell someone where a person is from.

This is a countable noun used in salons and in writing about beauty: book a Brazilian, get a Brazilian, the price of a Brazilian. It is a short form of the fuller name Brazilian wax, where Brazilian is the adjective naming the style. This short form is informal, so it belongs in everyday talk and salon price lists rather than formal writing. It is a private topic, and people normally use the word only where that topic is already open.