The Mosuo (also Na, approximately 40,000 people, living around Lugu Lake and Yongning in Ninglang County, Yunnan, and across into Sichuan) are culturally distinct from the Naxi and speak a closely related but separate Naish language. Chinese official classification treats them as part of the Naxi nationality, which is why population tables and language tables disagree about them. The cultural difference most often pointed to is the Mosuo's matrilineal household system; the linguistic difference is that Na and Naxi are not mutually intelligible.
The Chinese state recognises 56 nationalities and classifies the Mosuo within the Naxi nationality, while the Mosuo describe themselves as a distinct people. So an ethnic-population table counts them together, and a language table does not. Both are “correct” answers to different questions — which is why a page that says “Naxi speakers” without saying which is meant is not much use.
Naxi and Na/Mosuo sit in the same Naish group and share a border, a great deal
of vocabulary and a similar grammar, but they are separate languages: a Naxi speaker
from Lijiang does not follow a Mosuo conversation from Yongning. In language
documentation they are catalogued separately (Naxi is nxq; the Yongning
Na variety has its own descriptions and dictionaries).
The Mosuo are known for matrilineal descent and the “walking marriage” (tisese) arrangement, in which partners remain in their natal households. This is a Mosuo institution, not a Naxi one, and applying it to the Naxi is a common error in English-language writing about the region.
NaxiAI. “Naxi and Mosuo: what is the difference?” https://naxiai.com/naxi-vs-mosuo.