There were 10 people, altogether.
How many workers are in Tofugu?
I have three children.
When you see this on a number, for example ７人 or ３人, you're using it to count people. Just remember, both 一人 and 二人 have alternative readings. From 3 and up it's just the number plus にん, like this. ５人 is ごにん, １０人 is じゅうにん, etc. This is the counter for people, which makes this word the number of people.
The reading is the にん reading of this kanji. Just think of a number of people, all ninjas, standing there as you count them out.