As everyone said, it depends on the game.
Personally, I like to fill all 8 slots in a single faceset image. I make 5 generic expressions (neutral, happy, sad, angry, flustered), and then the other 3 are dependent on character (ex. smug character gets smirking expressions, jovial character gets laughing expressions, etc.).
The only characters that don't follow this are side characters that don't appear too much, or characters where you cannot see their face.