1) In a spot in the white space of the report, right click
2) Export > Entire Chart
3) You will see an export screen pop up. On the left side of that screen is the list of people in the report. Just above that list, on the right side of that list of people, you will see a [SELECT] Button. Click on that.
4) The number of the people in that report will be then be shown in the next screen at the bottom of the right hand panel.
Then, cancel out of that screen to get back to your report.
You can also set a filter at the filter button at the bottom of the index at the family tab and choose the exact same filter as you used in the report and see the number of people matching the filter.