This is a problem of direct relation or proportion.
The number of tomatoes to make the chili is proportional to the number of people in this way
5 tomatoes / 7 people = x tomatoes / 100 people
Solve for x: x = 5 tomatoes * 100 people / 7 people = 71. 42 tomates.
About 71 tomatoes and a piece (less than half) of other.
I think 71 tomatoes is enough, but she could use 72 if she wants to be 100% sure, given that her chilli is so good.