n is not specified, it is assumed to be one. The paragraph are
formatted relatively to the value of the right margin as set by the
RightMargin command. See RightMargin.
Do not ever ask me to implement justified paragraphing. I hate non-proportional justified text.
If you think paragraphing should insert ``smart'' spaces after full stops and colons, and do other such ``smart'' things, you should consider using a text formatter. TeX is usually the best choice.