I think something around that scale (say maybe 20 employees, but definitely not hundreds) was possible even before LLM got popular, but the people involved needed to be talented and focused. I'm not sure if AI will really change that though.

In 2014, Facebook acquired WhatsApp for $19B and they had 55 employees

Not necessarily. That's the old "I made Twitter in a weekend" joke.

That's not because you can technically replicate a product that your company will be successful. What makes a company successful are sales forces, internal processes and luck. Both are extremely difficult to replicate because sales forces are based on a human network you have to build, internal processes are either organic or kept secret, and luck can only be provoked by staying alive long enough, which means you need money.