If you use \pagestyle{empty} and you find some pages are
numbered anyway, you are probably encountering one of the style
decisions built into the standard LaTeX classes: that certain
special pages should always appear with \pagestyle{plain},
with a page number at the centre of the page foot. The special pages
in question are those (in article class) containing a
\maketitle, or (in book and report classes)
\chapter or \part commands.
The simple solution is to reissue the page style after the command, with effect for a single page, as, for example (in article):
\maketitle
\thispagestyle{empty}
or (in book or report)
\chapter{foo bar}
\thispagestyle{empty}
A similar technique doesn’t work for a book or report
\part command pages. For that, and for other detail, take look
at “getting rid of page numbers”.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=emptynum