Obviously you don't need all those people. Almost no company apart from like one-man companies or dying companies employ only the people they need. You employ as many people as possible that generate additional profit, short or long-term. Do you need to hire a conversion specialist? Of course not, though if she costs $400k a year while improving conversion by 10% the payoff is absolutely massive, and you'd have to be an idiot not to hire her.

I'm surprised people don't realize this, especially on hackernews.

That would maybe explain 500-1000 people.

What do you base your estimates on? What do you know about Spotify's business and what it takes it to run sustainably?