The main issue with multiple perspectives is if you have too many, you'll risk the readers not caring about your characters. This is fine if your story is plot-driven. Not so much for character-driven stories. Another issue is flow. Despite the novel being told from several characters, it still has to read like one story.
You'll need to have different voices in your head. I know how this sounds ;) If your novel has 10 different perspectives, you'll need to have 10 distinct voices or your readers will get confused.
You have your cast of characters. How do you handle changing perspectives without confusing or annoying your readers.
Listen To Your Story
I'm currently working on the second book in The Merging Worlds series. I had intended the female character to be the main voice. But, as I was writing, I realized I was more in the males' minds. Things like that are going to happen. It's fine. Write- feel it out and see what happens. You don't have to decide your cast of POV characters right away.
If there were rules to writing, Read and Write would be the only two. Find books you like that are told from multiple perspectives and study them. My initial problem with A Game of Thrones was that it was told from too many characters. That being said, this would be a great book to read on how to handle multiple perspectives.
I don't outline, but some people find it helpful. You could map out how long you'll be in this character's head and at what plot point. You could even layout why telling this part of the story from this character's perspective is best.
Most authors handle perspective shift by having one chapter dedicated to one character. They usually start the new chapter with the character's name so you know we're now in their head. However, I've read books where the author changed perspectives mid-chapter and it worked. This can be tricky, though. Most readers will assume the entire chapter will be in only one character's head. The best way to learn how to do this is to read, a lot. The Guardian by Sherrilyn Kenyon is a good example of changing POV characters within chapters.
Watch Out For Time
Let's say we start with John's POV and then switch to Mary's, spending 2 days with her. Then, we shift to Cat's and spend 3 days with her. You need to keep in mind that when we return to John, 5 days have passed for him. It gets worse when you spend months with one character. How to keep track of this? First of all, don't worry too much about it in the first draft. You'll only make your head hurt. Since my characters spend a great deal of time traveling, I plan to make a schedule for them during my editing phase. It'll have where they're going, why, travel time and how long they'll be at their destination.
While editing, make a "comment" or note about how long you spent with this character.
Let's take the example from above. When we return to John's POV, does it have to be at or after day 5? We're writers we can make anything work. Note, though, most readers will assume the story timeline is moving linear. One book I'm currently reading, Black Feathers, has characters from two different time periods. I didn't realize this at first. As I continued reading, the fact that the two characters were from different time periods became clearer. That being said, the story wasn't confusing. So, no, the story doesn't have to be chronological, but it can be tricky to pull off.
Technically, you only need to stay in one character's head for one scene. That being said, switching too often can be annoying. I didn't care for Bentley Little's The House because the author switched too often between characters. You'll need to feel out this one. Also, read. Learn what you like and what you don't like.
When To Switch
This is a tough one. Let's say, we've been traveling with this character for a number of months. They've been looking for something important, something that would help them figure out who they are. They've found that this something is behind a door. They arrive at said place and insert the magical artifact they almost died obtaining. The door opens.
This is a good place to end the scene. However, change perspectives at your own peril. Knowing me, I'd skip every scene until I found out what was behind that door.
Similarly, your character is fighting. They've been disarmed. The enemy is standing over them about to blow them apart with an energy ball the size of Texas. This is a good time to end a scene, not a good place to switch perspectives.
Let's say, the energy beam explodes in the enemy's hand. We don't know why. Now, this is potentially a good point to change perspectives. Depending on who you're switching to. If you're going to a character who has nothing to do with the battle then, I'd probably skip the scene. If you're switching to another fight then, I'd read that scene.
While reading The Warded Man by Peter Brett, one character was thrown down some hole, unarmed, with a monster waiting at the bottom. The chapter ended. As I turned the page, all I could think of was, "if the author switches perspectives, I'm going to be really angry." Fortunately, he didn't.
Again, the best way to learn how to handle multiple perspectives is to read. Here's a good place to start:
Goodreads: Books With Multiple Perspectives