Label legend interval with number of elements in ArcMap?
In layout view, open the legend properties and browse to the Items tab (see above picture) where you can specify which items show the count.
If you were asking specifically about showing the count in the Table of Contents, I'm not sure how this can be done. I looked around but only found this helpfile from ArcMAP that states
For example, you can use tags in dynamic label expressions, annotation, legend text, map titles, and in the values of fields used to label features. Tags aren't resolved by the ArcMap table of contents, Table window, or Identify Results window, so tags added to field values will be visible as tags in those windows.
There are 2 possibilities that I can come up with:
Use the script at this ESRI Forum; OR
Copy the numbers in the count column and manually paste them over to the label column when you define & tweak your symbology categories (as shown below).
