The Xiaomi Mi 8 was launched to mark the company's eight-year anniversary and named accordingly. Still, while a ten-year anniversary may hold more significance in the western world (see Apple's iPhone X), for the Chinese 8 seems to be it.
The number 8 is quite interesting. It's perceived to be the luckiest number in Chinese culture. It's because saying it in Chinese sounds really similar to the word that means "to generate wealth", as a Wikipedia article would readily tell you. Hence, people are used to paying skyrocketing amounts of money for anything from car license plates that feature at least one 8, to phone numbers that get progressively more expensive the more eights they incorporate.