It's scope that increases headcount, not users.
WhatsApp could scale to 900M users because the app they build didn't do a lot. It was for sharing plaintext messages between groups of people. That's quite hard at scale but you can solve it with 50 people as they demonstrated. When they started adding more features, that's when they needed more people.
If you keep the scope of your app small you don't need many people to build it.