Terrence Tao Measure Theory, hash symbol

Question

This is the first time I encounter this hash symbol. What is it supposed to mean?

Answer 1

In the online version, the explanation is in the next page.

Answer 2

As stated in the comments, the $\#$ is commonly used in number theory as an operator that returns the cardinality of the set, or the number of elements in the set (hence the use of the "number sign").