Although an inch is 72 points and twelve goes into 72 six times, that does not mean that you can really use six 12 point line to an inch. If you look at this paragraph, you will note that there is leading (space) between the lines. That makes the paragraph readable. How much space depends on the type face. The longer the risers (the part of the letter t above the letter e) and the descenders (the part of the letter p below the letter e), the less white space is needed between the lines. Some is always needed in case a riser would touch a descender or you have something in all capital letters. Most word processors default to two points of leading (they call it line spacing) for 12 point type. That yields five+ lines per inch.
