By some counts, only 600 of the 6,000 or ___ languages in the world are “safe” from the threat of extinction

On my exam I had this sentence:

By some counts, only 600 of the 6,000 or _________ languages in the world are “safe” from the threat of extinction.

The answer (fill in the blank) is "so", which I believe might be replaced with "less" or "more", since all 3 make sense. Can someone confirm this?

Your claim that less and more would both make sense is a difficult one to maintain.

The correct answer is so, as is illustrated by the original (even before looking it up, I was quite sure that this came from David Crystal).

Obviously, when we use so in this sentence, it just shows that the number of 6000 is an estimate, and that the real number of languages around the world is not exactly known.

Now, what happens if we use less?

By some counts, only 600 of the 6,000 or less languages in the world are “safe” from the threat of extinction.

There is a big semantic issue with this sentence. The, quite clear, overall message of the sentence is that only a small part of all the languages in the world will eventually survive. By downplaying the total number of languages (6000 or less), we actually contradict that the number of “safe” languages is only a small part.

Grammatically, the sentence is acceptable (although strictly speaking it should be fewer!) but the semantic contradiction (small amount of total <-> small total amount) ensures that the sentence would never be spoken or written by any fluent speaker of English - unless their aim was to confuse the audience.

Now, what about more? Sure, then the sentence would make sense:

By some counts, only 600 of the 6,000 or more languages in the world are “safe” from the threat of extinction.

If you answered more, you could defend it - provided that no more context was given. The meaning of this version is distinctly different from the version with so.
By saying there are 6000 or more languages in the world, you make a hard claim that there cannot be 5999 or less, and that is a claim that the author is unlikely to (want to) make, especially since he indicated earlier in the text that the total number of extant languages is an educated guess at best.

Also, using or so indicates that he might think there were maybe more than 6000 languages until recently, but the number might already have dropped to somewhere under that number - his message is that languages are dying fast!

So, if you want to challenge the question (or rather, the accepted answer), you better make sure that you do not propose an option that causes confusion. All in all, I would assume that an exam like this is about measuring your understanding of the language, and your ability to distinguish (subtle) differences in meaning, in context. In that sense, the point is not to provide an answer that is just grammatically correct, but to give the best suited answer that makes the most sense in context.

In this sentence, so is the option that makes most sense, more can be defended, but I would not even try to defend your choice for less.