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.
I realized recently since my game is still a turn based game, that something like the card game Yomi is a lot closer to what I should try to aim for combat wise. Should also help for those who don't really play fighting games to make things a bit easier to understand. ugh the struggles of trying to show love for my 2 favorite genres in 1 go lol.
Holy crap, it's cold. Our heat went out this morning and the repair guy won't get the part needed to fix it at least until tomorrow. Had to be now of all times, when we have stuff to do all week and it's the coldest night of November so far.