There are about 800 unique weekly committers to the Chromium project, so that's a start at gauging the number for that project. A little harder to find that same figure for Firefox, but Wikipedia says Mozilla Corp had about 750 employees as of 2020.

Anyway, if you have $50M, you can afford 500 people at $100k, or 250 people at $200k. So you simply declare, this is how many people it takes to make a browser, and set your goals and timetables accordingly. I feel like the goals and direction might be more important than the number of bodies you throw at it, but maybe that's naïve. But when the product is mature like Firefox (or Chrome for that matter) you do have some flexibility on the headcount.

Brave has about 300 employees and don’t break out engineers [0]. One of them is Brandon Eich so that counts for a bunch.

Their revenue is only $52M so kinda what Mozilla would earn off their endowment.

[0] https://getlatka.com/companies/brave.com

Ladybird had fewer devs, so what were these devs at Vivaldi doing?

I don't think your argument has a lot of merit. 28 is not a magic number.