Symbol for the cardinality of the continuum

I have seen the use of $\aleph$ and $\mathfrak{c}$ as well the explicit $2^{\aleph_0}$.

If you're uncertain, it's best just to add "We denote the cardinality of the continuum by ...".

(Edit: I should perhaps clarify, that $\aleph$ is not uncommonly used in Israel in basic set theoretic courses. Some of which are taught by respected set theorists, although not all of them. It is true, however, that in a more advanced capacity this usage disappears.)

I have never seen $\aleph$ used without a subscript in any treatise of Set Theory.