"The browser ensures that cohorts are well distributed, so that each represents thousands of people."

> how many cohorts can a person be part of?

It's just:

    cohort = await document.interestCohort();

(Disclosure: I work on ads at Google, speaking only for myself.)

"Additionally, a subset of low entropy variations are included in network requests sent to Google. The combined state of these variations is non-identifying, since it is based on a 13-bit low entropy value (see above). These are transmitted using the "X-Client-Data" HTTP header, which contains a list of active variations. On Android, this header may include a limited set of external server-side experiments, which may affect the Chrome installation. This header is used to evaluate the effect on Google servers - for example, a networking change may affect YouTube video load speed or an Omnibox ranking update may result in more helpful Google Search results." -- https://www.google.com/chrome/privacy/whitepaper.html

And my personal browsing habits are for me, that doesn't seem to stop Google from inventing new and imaginative ways to use them for ad targeting. The id is also long enough to be unique in a group of thousands, which is a size range in which the groups created by this new way of targeting ads apparently are.