For example, I'd read a first chapter of a novel draft a few years back. Eighteen characters were introduced in the chapter, two protagonists, a host of antagonists, some in the frame of the story, others outside of it, apparently less focal characters were introduced as well. I didn't know who was who after multiple reads. They were all named and the names carried what little characterization that was developed.

It was busy for a first chapter. No idea which of the multiple trains of thought were central, which of the characters was central or which protagonist was primary. I couldn't discern what the story was about or which direction it was going in. It left me confused.

You don't have to call the midwife "the old woman", the midwife is fine--I could even stomach a Mrs. Bailey or Joan if someone relevant to the story knows them. If not, then it would be easy enough to have a character avoid addressing her and leaving her just "the midwife". But, like I said, it depends on certain factors.

If you're beginning a stand alone (that's really the first of a series), you may want to leave other characters to trail off on in the next story--in that case you might name them, but use only their first or last name for reference while it's not their (or someone close's) PoV.

Is it really too many names? Too many similar-sounding names? Names that are seldom used, so hard to remember? Too many characters to keep in mind? Characters who, once they leave a scene, become forgettable--then reappear and one's forgotten who they are? Characters who seem the same, or generic, without distinguishing features that help the reader remember them? Too many characters one simply doesn't care about?

Set in a small village or hamlet (up to 100 people), everyone knows everyone else, everyones elses business, and how often you hang the laundry out, and what you had for tea last night

Set in a town (up to 300) people, or in the same street. They will know your first name, not necessarily your surname, they will know your profession (fantasy setting or SF) some will only know you by sight. They will know that your daughter Chloe goes to school with their 4-times-removed cousins daughter Alice.

Set in a city, some know your first (work colleagues/close neighbours)or last name, or just by sight.

Healers, Blacksmiths, Mayors, shop owners, and inn keepers will always be known by name in the village or town in a Fantasy setting.

If your minor characters remain (1) memorable in their own right, (2) have their own backstory no matter how brief their appearance in the story is, and (3) are all distinct from each other in name, appearance, and function in the story...well, you should be all right.

1. Too many characters introduced all at the same time. This is particularly easy to do at the beginning of a novel, since every single character is necessarily new. It is hard at that time to tell important from unimportant characters since you are limited in the amount of "introductory" space you have. I've found it best to keep the introductory chapters as "private" as possible, in the sense that I only have one or two main characters in there, and most of the interaction is between them with only cursory mention of other characters of interest. Extras are rarely mentioned by name, even if I intend those extras to be named persons later.
2. Unimportant people are introduced too thoroughly. If "Jill from down the block" exists in the story only because her daughter is a witness to the kidnapping, but she does nothing particularly important herself, it may be best for her only to be "Jill from down the block," as opposed to "Jill Tannen, the short, out-of-shape woman who always stays at the back of Susan's yoga class. Her husband was often out of town, and she spent most of her days reading the paper on her porch. Susan once thought that to be a sad life for a thirty-eight-year-old woman with two older children, so alone all the time, but nowadays she almost envies the woman her privacy."
3. You are accidentally writing an Ensemble book, with a cast of dozens, all of whom are important. The best remedy for this is either to merge characters or, if you want to be soap-opera-like about it, kill lots of people.
   

The number of proper names characters that a reader can tolerate is directly proportional to the amount of plot that passing during their use. My rule of thumb is to have no more than two proper names in one paragraph. Sometimes I must break this rule, so I remind myself that "this paragraph will take extra effort from my reader."

I will use no more than eight proper names in a chapter, twelve in a short story, eighteen in a novelette, and twenty-four in a novella. For a novel the sky is the limit, as long as I do not violate the eight-per-chapter rule.

I once read a writing book that suggested starting each character's name with a different letter. There are only 20 or so characters that you can have before this gets mighty tough, but I think a better rule of thumb is not to have the names look similar or sound similar. I would never have a Julie and a Jessie, but a Julia Smith and a Juan Carlos Gutierrez can coexist easily. Particularly if Juan insists on being called Juan Carlos. Nonetheless, I wouldn't make Jula and Juan Carlos my two main characters b/c of the first-letter-collision issue.

This accidentally happened to me with last names. I had one family with the last name Keeney, and it was important for another character to be named Kearney (an unwanted child of the family, as it turns out, so he got a bastardized version of the name). I didn't realize for a while that I had also introduced a minor character by the name Kelley in the second chapter (a Keeney and the Kearney were both in the first). To top it all off, a fairly major character (introduced in chapter four) had the first name Keegan.

I ended up leaving Keegan alone because the characters don't address each other by their last names, for the most part (one noteworthy exception), and Keegan was almost never around Mr. Kearney and Mr. Keeney. In chapters where they were together, Grayson Kearney goes by Grayson, Keegan Thynne by Keegan. Mr. Kelley got his name changed to Mr. Morgan. And that's the tale of all my K's.

One thing you can do is tag people with some kind of memorable quirky identifier. Like, James with one ear lower than the other, or Robert who wears a red tie no matter the color of his shirt, or Vladimir with six kids and a funny accent that you main character can't place.