This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well. Just be sure to take note of the rendaku for the second half, where すう goes to ずう.
What’s the total number of people going to karaoke?
What are the recruitment numbers for the new Tofugu writers?
There are a great number of people who disagree with my proposal that everyone in this country must wear the same pants.