Numbering paragraphs in word is fairly easy using styles. Let us say that you have the style SPECIAL which is applied to every paragraph. It will be based on the normal style (or none) usually. Set up a new style called say PNUMBER. Dont set any characteristics for it except the numbering you want. Set your other style SPECIAL to be based on the style PNUMBER instead of normal (or none) and all your paragraphs will inherit it. The will be numbered sequentially starting from 1 to the end of your document (or according to the PNUMBER style).
If you want to number every fifth para I would guess you are looking for something that is unobtrusive. Well, in Word 2003 for example, there is a 'font' button tucked away in the numbering style itself. Click on this and you can set the font characteristics separately for the number of the paragraph. You could, for instance set it to be grey and very small (6 point size). This would be very unobtrusive alongside your text.
If you still really need to only number every nth para, then you'd probably want to use a macro to apply a style to every 5th line, so you'd need a SPECIAL and a SPECIAL2 style say, to alternate between.