These are hard questions. Why? Because there is no one right answer.
If you're writing a mystery, there should be enough information in there that a sharp reader can figure out who did it. There's nothing that mystery readers hate more than the author pulling a fast one--meaning, not giving them the information they need. This doesn't mean you need to beat it over their head.
I don't write straight mysteries, but my readers learn the important facts with the hero and/or heroine as they investigate the crime. Sometimes, they learn something that they don't realize is important until later. Or someone lies to them, but they don't realize that until they learn additional information. There's so many ways you can write this. In romantic suspense, the mystery aspect is usually secondary to the suspense, and usually savvy readers know who the bad guy is.
I was once lambasted by a reviewer because they felt I didn't give any information as to who the killer was in one of my books, and just pulled the guy out of thin air. The ironic thing is I thought I gave too MUCH information. I went through the book (because I'm neurotic--most authors are--and feared she was right.) I found six places that gave clues as to who the killer was, but those clues had to be put together to see the whole picture. The killer was a cop, and there were three cops who were secondary characters to the story--herring part was (I thought) who the guilty guy was, NOT that he was a cop which was pretty obvious.
You don't want to hand-hold your readers, but you want everything there somewhere!
In terms of too many characters . . . there should be enough characters to tell the story. Some stories will naturally call for more characters; other stories won't. I tend to write with a large cast of characters, but I do take some out in edits if they truly don't contribute to the story.
For example, in the book I just finished writing, I had a character--my heroine's brother--whose sole purpose (not intentional!) was to introduce the hero to the heroine. I didn't realize that, but I'd painstakingly put him in scenes where he really had nothing to do, and he sort of got lost. I took him out and everything worked beautifully. He was still (indirectly) the catalyst to bring the hero and heroine together. It was hard, though, because he's a favorite character! But I'll bring him back
Another example is that in the same book, my editor felt that I had too many characters introduced too early, and that it would be hard for the reader to keep track. So I went through and cut two characters from chapter three which was a briefing scene in an FBI office. The characters still come in later, but they didn't have to be there so early. The chapter still worked well, and there was less to keep track of.
So when you're editing, consider whether another character can take on some of the rolls of secondary characters who don't seem to add anything. I'm not talking about the people your characters meet in the course of an investigation--I love having those one-scene characters who walk in and the hero learns important information. Think LAW & ORDER. Elliott and Olivia go interview a guy, he gives important information, they go find a dead body. You never see the guy again, but you NEED him because how they find the information is important.
I hope that helps!