Baldur's Gate 3. The default party size is 4. In single player it's filled with NPCs. Might be a long commitment, but it's a bloody good game. If you are more than 4 you can increase the party size with mods that can be installed from within the game. You could increase the difficulty to compensate.
Incidentally the first two games can be played in multiplayer as well, with up to 6 people. But although they are awesome as well they might be a little bit dated.
If you're looking for F2P Path of Exile is one of the best. It's an action RPG like Diablo (which is also great in multiplayer).