In Finnish, elative can also express ''a part of a certain quantity''.
30% saksalaisista puhuu hyvin englantia. (30% of German people speak English very well.)
Kaksi teistä. (Two of you.)
*Since the subject is a group of/ a part of something, the predicative verb is always in the 3rd. pers. singular. (Just like partitive case.)
In your example, ''yksi + elative plural = one of the...''. (Note that if an adjective is added before the noun, you have to follow the agreement, that is, adjective, pronoun or numeral in front of the noun is in the same case and number as the noun.)
I'll give you some examples:
Saara on yksi tunnetuimmista laulajista. (Saara is one of the most famous signer.)
Tunnettu > famous, tunnetuin > the most famous, yksi tunnetuimmista > one of the most famous
Oulu on yksi maailman kiinnostavimmista paikoista. (Oulu is one of the most interesting place in the world.)
I am not native Finnish speaker, any correction from native is welcomed!