Rotokas, spoken in the mountains of Bougainville, Papua New Guinea, boasts one of the world’s smallest phonemic inventories with just 11 phonemes.
For comparison, English has 44 phonemes. A phoneme is generally regarded as “a set of speech sounds that are seen as equivalent to each other in a given language”. So even if the “k” sounds in the English word “kill” and “skill” are not exactly identical, we see them as equivalent within the language, and that’s why we call it a phoneme.