Switching character viewpoints? (how often is to often)?

You can write with an omniscient voice - that it, the narrator knows everything, every thought, every mood of every character. That is how most older novels were written, but today, it is more common for writers to choose one character to tell the story through. If, for example, you're writing in the first person (I...) then the only voice you have is that character's voice. Some writers switch viewpoint with chapters or sections. But there is nothing wrong with an omniscient voice.