Formal:
* Muitos/Muitas: This is the most common and straightforward way to say "a lot" for both masculine and feminine nouns.
* Bastante: This means "enough" or "sufficient," but it can also be used to mean "a lot" in certain contexts.
* Grande quantidade: This literally means "large quantity" and is a more formal way to express "a lot."
Informal:
* Um monte: This means "a bunch" or "a lot" and is a more casual way to say it.
* Um bocado: This literally means "a bit," but it can also be used to mean "a lot" in informal contexts.
* Muita coisa: This means "a lot of things" and is a more specific way to say "a lot."
* Pra caramba: This is a very informal and slang expression that means "a lot" or "very much."
Examples:
* Tenho muitos amigos: I have a lot of friends.
* Ela come bastante: She eats a lot.
* Há uma grande quantidade de livros na biblioteca: There are a lot of books in the library.
* Eu tenho um monte de trabalho para fazer: I have a lot of work to do.
* Ele comeu um bocado: He ate a lot.
* Tem muita coisa para fazer hoje: There are a lot of things to do today.
* Ele está apaixonado pra caramba: He's very much in love.
The best way to choose the right expression depends on the context and your audience.