Would you say the following is generally true, especially for an inexperienced writer like me:
If POV shifts have to occur at all, they should be in separate chapters.
Mostly, usually, but there could be exceptions, theoretically. I'd say that's generally a decent guideline.
The fewer POVs, the better.
Put it this way--the more POVs you have, the harder it is to get the reader oriented and invested in the characters, and the more likely it is that you're inserting stuff that doesn't need to be there. But I wouldn't say it's a rule to use fewer POVs.
Omniscient POV is the hardest to write and least liked by readers.
Disagree. Some readers don't like it, I hear, but many like it just fine. Many of my favorites are in omniscient. POV isn't the problem; it's how it's written. Please don't think you can't write omni; but do learn how to write it by reading a lot of it and seeing how it works when done well.
Nothing is a hard rule; nothing is set in stone; there is nothing that you
can't do. But certain things are harder to pull off, from what I've seen. (And tried.)
Here are the potential issues when writers attempt multiple POVs, as they occur to me:
1) The more POVs there are, the less time you spend with the characters, and the harder it is to get invested in them.
2) The more POVs there are, the better the chances are that they become generic or indistinguishable or plain boring.
3) The more POVs there are, the greater the chances that the writer hasn't spent much time developing the characters, sometimes reducing them to two-dimensional lists of traits or cardboard cutouts.
4) The more POVs there are, the greater the chances that you're including scenes that are not necessary.
5) The more POVs there are, the greater the chances that the writer has a general issue with overwriting (can stem from a desire to show every little thing instead of sketching certain things more broadly, letting a reader's imagination fill in details, etcetera, which bleeds into the writing more generally).
6) Switching POVs too often or between too many characters is confusing and disorienting, which becomes tiresome and frustrating.
7) Omni is not an excuse to write in infinite POVs. It's one POV--that of the narrator. Too often, though, it's treated as the former.
And that's all I can think of now, although I'm sure more may come to me...
If you can avoid these things and the story benefits from it, then by all means, use more than one POV.