The answer to your question depends on whether you're talking about page
numbers, paragraph numbers, or something else.
Page numbers are easy: a { PAGE } field can be placed anywhere on the page.
The simplest approach (though I don't recommend it for a book) is to use
Insert | Page Numbers and choose the Outside placement. The numbers are
inserted in frames, which can be dragged anywhere you like. If you insert
the numbers in the actual page header, you'll need to check "Different odd
and even" on the Layout tab of Page Setup. You can put the numbers in the
margin of the header area by giving the header paragraph a negative indent
on the appropriate side; if you need them to be in the side margin lower on
the page, you'll need to put them in a text box or frame.
If you want paragraph numbers in the margin, just give the paragraph a
negative first-line indent (or negative indent if it's a heading style).
If it's some other kind of number you're talking about, see
http://home.earthlink.net/~wordfaqs/MarginalText.htm for some other
suggestions. Note that if you end up using a SEQ field, you could save a SEQ
field in your framed style as an AutoText entry.