The book Web Browser Engineering may be interesting: https://browser.engineering/

- how many people in a team do you need and how long would it take to write a browser engine from scratch

The boring not-really-an-answer is "it depends on which sites you want to render properly": a browser capable of rendering Hacker News is a perfectly fine one-person project (which the book would get you to), supporting things like banking and online mail requires not just developers but also people willing to "dogfood" test it as their daily browser