What does the symbol $ \#$ represent in mathematics?
For example,
In J. Neukirch's algebraic number theory book (page $171$), it has been mentioned that-
the number $\# W_v$ of extensions equals the index $(G:G_w)$.
where $W_v$ is the set of extensions of valuations $v$ and $G_w$ is the decomposition group of automorphism.
What does the symbol $\#$ represent here?
Does this mean dimension or number?