so ['fɔɪ.bɫ] (phoible) has a list of phonetic segments (sounds) used in all the world's languages. There's approximately 2,160 segments.
1,800 of them are in less than 1% of the world's languages.
Only 19 of them are in more than 50% of languages.
the average number of segments in a language
I downloaded the phoible database of language inventory sizes, and wrote some code to calculate the average number of segments in a language:
35.2125
the average number of segments in a language
it's important to note: this is only according to what's in phoible's database, it's not a definitive answer (especially since defining segments is.... harder than it seems)