Have you ever been at a party or professional event where you have met a small group of the attendees some time back so you barely remember them, and there are dozens of other people attending as well?
You stood there with a glazed look in your eyes as you struggled to remember the names and relationships of the people you've already met while even more people are introduced to you, and you have to figure out how these people fit in with the first group.
A nightmare, wasn't it?
Yet many writers forget how hard it is to keep up with characters in a novel. They insist on starting the novel with a group scene in which all the heroine's coworkers are introduced. Each character enters the scene, does a little song and dance so you have some idea of who they are, then the next one enters and does the same thing. By the fourth or fifth character, the reader is in shell shock if she's still reading.
Then, the novel opens up, and even more characters are introduced.
Other writers of series, particularly paranormal romance series, have an ongoing group of characters--usually the happily married heroes and heroines of past novels who have to have a cameo or minor role--as well as the new hero and heroine to include with their short term bad guys and minor characters, but, wait, the author really wants you to meet the half a dozen new hunks waiting for their own novels, heroines, and happily-ever-after as well as the bad guys waiting in the wings for their comeuppance.
Some readers can keep up with all these people, but most of us can't. We reach a point where there's so much character clutter that we can't connect with the major characters and the main plot so we close the book and vow never to read another of them.
How do you escape this cast of thousands syndrome?
First, you must realize that while you spend many months with these characters and know them very well, the reader won't.
Keep the introductions to a very few at a time. Secondary characters should only be introduced when they are needed in the plot. Those officemates of the heroine may play big parts in later books, but only the wacky receptionist who will introduce the heroine to her new love interest and play clumsy matchmaker will be needed in this book so only she should be introduced.
As great as the other characters are and no matter how eager you are to introduce them, don't.
If you have characters from other books, don't bring them back unless they serve a specific plot purpose.
If you have new characters for the next book in the series, don't put them in unless they serve a very specific plot purpose.
If you are lucky enough to have readers wanting to know how Lance and Patty from your first book are doing and whether their baby has been born, you can write a short story or novella about them as a freebie on your website. Fans love that.
Many of us don't love the author tossing these former characters into the current novel with no other reason than to please a few fans.