Few hundred or thousands of users can run happily off one server. Then you can just... buy a bigger box for long time, maybe not need more for hundreds of thousands of users.
And once you put all of the effort to get highly available loadbalancing, a bunch of app servers, maybe use some distributed database or add a lot of caching for your current one you can, again, just add more and more nodes for a long time till you get to "next step".
Of course, make your app in Rails with people that have no clue about architecture and some N+1 queries and you might need to get to the scaling steps much earlier
> Imagine 50 "bricks and mortar" engineers trying to provide anything to 900M users.
They are not providing any support to the users tho. Hell, I'm pretty sure some of the best selling cars and motorcycles in history(...being probably beetle and honda super cub) had less than 50 engineers