How much do you know about the machinery running that single website? If you can't answer this question then the reduction in your assumption of how simple it is falls flat.
How many large orgs with hundreds of millions of users have you worked for? Derive from that experience how many people it takes to run their systems...
I've been working on a single website for a Fortune 50 company more than 20 years. For the first 12 years years it was just me. I built the entire thing from the ground up. Provisioned servers, installed software (database, web server, application server, etc.), hardened software, optimized software, designed database schema for hundreds of tables, wrote tens of thousands of sql scripts, optimized queries, wrote server side software, wrote apache config file, built front end, etc. After 12 years they hired another developer and the two of us have been the only ones working on it since. Now, our website isn't used by hundreds of millions of users, but it has been used by millions of users and we get about a million unique users each day. Obviously it will take more than 1 or 2 developers to create and maintain a website of Twitter's size, but 10s of thousands of developers? That seems pretty crazy.
How many large orgs with hundreds of millions of users have you worked for? Derive from that experience how many people it takes to run their systems...