Nermander makes a very good point (pardon the pun) about different fonts being different sizes at the same point-size.
You'll also need to take into account the thickness of the characters. "Thin" fonts will take up less space but they can be difficult to read for some people when parts of the characters are too close to each other (such as the verticals of an "m"). And "fat" fonts can be difficult to read as body text full stop.
And, one other thing I forgot to mention is that how you get your document printed makes a big difference too.
For instance, text that has capitals only 3mm high might look great and be easy to read when it's been printed on glossy paper on a professional machine but it might be very difficult to read when printed on a much-lower-resolution home/office printer.
Your best bet is to experiment.
Sometimes it's better to leave out some information than try and cram everything onto a page that becomes a chore to read because the text is so dense.
A good idea would be to look at various newspapers and see how they do things.
They've been honing their designs for decades, if not centuries (in the UK "The Times" has been printed since 1785), and they're very conscious about keeping the number of pages to a minimum.
For example, "The Sun" in the UK has a daily circulation of just over 2 million. So, if you're printing 2 million copies a day, losing one sheet - 4 pages - per edition saves you from printing nearly 2 million sheets a day. It all adds up!
P.S. Have a look at this Times front page from 1788
http://en.wikipedia.org/wiki/The_Times#mediaviewer/File:Times_1788.12.04.jpg to see an example of what dense text looks like.