Out of curiosity, does Google really have that many engineers working on Chromium? According to Wikipedia, Mozilla has ~750 employees and only a subset of those are working on Firefox.
And remember some people might be working on experiments/research/ideas that never make it into the repo.
And there are lots of libraries in other repos that kinda only exist for Chromium that Google also works on - eg. the Skia graphics library.
But... on top of those engineers will be UX designers, managers, HR, finance, chefs, and all the other people who make an organisation work.
I think 1000 is a good guess - maybe even 2000.