Think it is 10 player co-op max.

Co-op PvE (2-10 players) - - Whether you want to brave the lands alone or venture with trusted allies, Valheim supports independent, player-hosted servers and unlimited world creation. We recommend playing co-op with 3-5 players.

Yeh 6 is about max, unless you're all really close or on same network. Get's abit laggy after 6. But I have been on servers with 10 and it is fairly workable, but it is also v laggy.

Thanks for your unswers. guys