六 sounds like 溜 ('smooth, flowing'), making it a lucky number associated with things going smoothly. '666' is internet slang for 'awesome / skilled' in China — the opposite of its Western connotation.
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六 has a rare second reading 六 (lù) used only in a handful of place names: 六合 (Luhe District of Nanjing) and 六安 (Lu'an, the prefecture in Anhui famous for 六安瓜片 tea). Treat these as fixed proper-noun exceptions — everywhere else, 六 is liù.
Bottom radical 八 (eight, splayed lines) is the indexing radical of 六. Even though etymologically 八 has nothing to do with 'six', the splayed-line shape is unmistakable beneath the lid. Useful pedagogy: think of 六 as 'a small lid over a splaying-eight' — the indexing radical that lets you find it in any traditional dictionary.
The 亠 'lid' radical — a horizontal plus downward dot used as a graphic cap on many chars (亡, 京, 文). Here it functions as a positional cover above the splayed legs below to form the 'six' silhouette. Strict etymology says 六 originally pictured a small hut.
