On the layout, you can add a Dynamic Text element. Choose Table Statistic, and when you choose your points layer, you can select the Query Map series rows. This will update whatever statistic you choose based on the features which fall within the map series feature
In the map series options, there is also a box for Clip to index feature, which will only include features in the subject feature. It's not required for the dynamic text to work, but may help your layout, as it will hide any features outside of the feature which are not included in the count. Otherwise, the visible points and the count may not match.
- Josh Carlson
Kendall County GIS