Indicates that there is no restriction on the number or identity of people; anyone or any number of people (may do/attend/etc.).
パーティーは何人でも参加できます。
Anyone can attend the party.
会場には何人でも入れるわけではない。
It doesn't mean that any number of people can enter the venue.
このサービスは何人でも無料で試せます。
This service can be tried for free by anyone.
彼は何人でも気軽に話しかけるタイプだ。
He's the kind of person who speaks casually to anyone.