All depends on the character. I personally don't usually use expressions at all, though I once had a team make me create 20+ expressions for each character (there were 18 main characters in the game, so do the math). One of many reasons I left that team.
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.
I'm not really a fan of expressions, really! I'd rather have strong dialogue than a gazillion faces, 'cuz no matter how hilarious yo expressions, they don't make up for weak dialogue, not for me anyhow...
I love portraits tho! So I'm with @HexMozart88 on this one, just one is enough!
Also yea, what @ATT_Turan and @arsMori said, think about what ya need... Aaaand, on that note, many expressions can fill a lot of roles, like if you have a frowning face, it cud be anger, or concern, or concentration, you name it, depends on the context!
Yeah I can't really answer as I make each expression to fit the game dialouge. I also make the sprites reflect those expressions.
So depending on the character it can be 10 + expressions to just one face portrait.
My advice is write your game story first so you have an idea what expressions you need so you don't end up wasting time.
There's 7 main areas in the game that are all connected. This train get's you from an underground lab to the center of someone's subconscious. inside the train you can talk to some of the passengers. but going to the top of it, there's battles.