At least 40% of the characters in my novel appear for only 2-3 chapters and I don't really have the chance to develop them. Is this too many? How can you determine you have too many characters appearing for a very short duration?
I have characters in every chapter that have personalities, and often significant and lengthy communications with the main character. They don't appear again, they are basically in the place where the MC meets them, and she is on a journey or quest of some sort. So she talks to them for awhile, they may show her something, she may do a favor for them.
I think this is realistic. People know other people, they can have conversations with them. When I am out in the world (not chained to a keyboard) I know by name the waitresses at the coffee shop I go to, for some of them I know the names of their kids and their ages. Same with my neighbors.
It would be strange, I think, to go on a quest and not have any conversations with the locals. Half the time, you are explicitly looking for somebody that knows something, or trying to find records with somebody that keeps them. You're not traveling from Miami to Chicago for nuthin', there is someone or something in Chicago you need to see, and if it is something, there is probably someone there guarding it, or keeping the records, or that can help you understand the something.
The more my MC has to interact with a character, the more time I invest in thinking about that person's personality and attitudes, and how I can create conflict with my MC to keep the story from getting boring. (You can't just deliver the information, you need to make it hard won.)
Don't worry about walk-ons, or characters with a limited window of appearance. Make them realistic in proportion to their importance and time "on the screen".
Not every character that you think should be naturally important is actually important. If you watch "Sleepless In Seattle", notice that Meg Ryan is actually engaged to some guy for most of the movie, and in the end dumps him. Analytically you might think that her fiancé should be an important character, but he gets very little screen time and is pretty much a cardboard cutout, he only exists to create a conflict for Meg (or a more dramatic moment of decision and commitment when she decided to dump him). The same can go for parents, siblings, lovers, etc. An important character influences the MC and their course (positively or negatively), even if present for only half a chapter, or a few chapters. Important characters should feel like real people. Walk-ons and Extras don't need much development, they are necessary for realism (as Rasdashan says, the E.R. isn't empty) but don't need any personality; there are four men at a table in the bar playing nickel poker to pass the time.
I have a large assortment of walk on characters who appear briefly. Some of them might return, but most will at best be referred to by another character and just disappear.
I find this realistic and reasonable since no one walks through an empty town.
For example, let’s say your MC is injured and taken to an ER. You will need a few nurses, a couple of doctors, some other patients waiting - maybe a few with friends or family. Most of them are wallpaper, atmosphere just so you don’t have the bizarre situation of going to an empty ER. After your MC is treated and released there will be no real reason to bring any of those characters back, but the scene plays better if they are still real characters with lives they will live - just glimpsed for a moment.
Having a large cast of characters adds dimension to a work and some can be sources of levity or wisdom.
This is not to say that a novel cannot be compelling with just a few characters, but it limits one’s options.
I think of my walk ons as the seasoning in my story - too much and it just needs help, not enough and you have a bland story - a fine balance adds dimension and flavour.
I hate to give the old standby answer, "it all depends ..." But, it all depends.
Of course in most stories there are characters who appear briefly to perform some specific function and then disappear, and the reader thinks nothing of it. Like if the main character rides on an airplane, you might briefly mention that the pilot came over the PA system and announced that the plane is arriving in Chicago where the local time is, etc, as a device to say where the hero was going. The reader is not baffled or disappointed if the pilot is not a fully developed character and does not turn up again later in the story.
Some stories are naturally episodic. For example, in many detective novels, the brilliant detective investigates some crime, solves it, and then moves on to the next crime, one after the other. The reader expects characters related to each crime to appear, to be extremely important to the story for a chapter or two, and then to disappear, never to be mentioned again.
But in general, the reader expects their to be a cast of characters who move through the story together. While it's true that in real life we make friends and then move to another town and never see them again, we get a job and get to know our coworkers and then get a different job and have a new set of coworkers, etc, in fiction we expect more continuity than in real life. In real life I have years to get to know these people before I move in. In a novel there may be a few pages. Characters who come and go too quickly don't have a chance to grab the readers attention.
I read a novel once that consisted of a series of episodes aboard a star ship, where the heroine met people who were traveling on this ship, learned they had some problem, solved their problem, and then the characters disappeared from the scene and she met new people with new problems. While it was a good story, and I cared about the heroine's problems, I found that I didn't care much about the minor characters' problems. They were introduced and a page or two later, when I had not had time to "get to know" them at all, suddenly the writer expects me to care about their personal problems. I just didn't. They weren't around long enough. If my brother tells me that he's having problems with his marriage, I care because I've known him for decades. If a stranger walked up to me on the street and started telling me about his marriage problems, I'm sorry but I just don't care.
That's not to say that every character has to be around for the entire story. But they have to be around long enough for the reader to get invested and care about them.
Oh, and remember: The characters can be much more interesting to the author than they are to the reader. You've spent months thinking about this character and developing this character and getting into the life of this character. But to the reader, if it's just a few pages, he's only spent minutes.