This area is thought to contain more uncontacted tribes than anywhere else on the planet, with an estimated 2,000 individuals across up to 20 isolated groups
It means someone didn't have a proper editor look it over before publishing. It's two separate phrases, merged:
across 20 isolated groups
up to 20 isolated groups
The first expression means spread out between 20 groups. The second means at most 20 groups.
Put them together and, as you found it, it's confusing even for native speakers. Assuming we don't really care about the exact number of groups, I would have instead written:
... an estimated 2000 individuals across 20 or so isolated groups.