Only when you want a new paragraph
On manual typewriters, the carriage was the part on top that held the paper and scooted leftward as you typed. At the end of each line, you’d push a lever to move the carriage to the beginning of the next line. On electric typewriters, this lever became the carriage return key, which you’d press at the end of each line.
The terminology has stayed with us, but on a word processor, you only use a carriage return to start a new paragraph.
As with the word space, use only one carriage return at a time. It’s common to see multiple carriage returns used to add vertical space between paragraphs. Bad idea. If you want vertical space after a paragraph, use space between paragraphs.