Re: Horror scenario
You don't need that many people, but remember to budget in:
1. Extra payments for every worker, because salary is not the only cost to the employer.
2. Translators to all the supported languages.
3. Testers for every supported operating system, and yes they have them.
4. Separate development teams for every operating system, because they have those too even if they don't necessarily need to build OS-specific versions of every part.
5. People to manage the build system, IT, HR, finance, for all the programmers/translators/testers.
6. People to either write or get licenses for all the extra software you need to have a modern browser, like fonts for every language. There are many more components involved.
7. Security teams to try to find vulnerabilities and to fix any that someone reports quickly because a browser is a frequent target.
And the optional ones that modern browsers all have:
8. Researchers developing new networking, security, and web standards. You'll need some people to implement them when others do, which is made a little easier by working with them to create them.
9. People who add additional features that you don't care about but other people do. I'm guessing that a lot of people have never used, for instance, Firefox's new offline translation function, but I do and I'm glad they built it.
10. Management, which you will need at least some of, even if it's only taking the most administration-capable programmers off task at times so they can make sure multiple people aren't writing the same thing or the testers are testing the thing that needs a lot of testing.
11. Probably a lot more types of people I didn't think to list or should be split into their own categories.
Not everyone who develops something is a programmer.