Simple documents (those that start at page 1, and don't have any
breaks in their page numbering until their last page) present no
problem to the seeker after this truth. The number of pages is
reported by the lastpage package in its LastPage label.
For more complicated documents (most obviously, books with frontmatter in a different series of page numbers) this simple approach will not do.
The count1to package defines a label TotalPages; this is
the value of its copy of \count1 (a reserved TeX count
register) at the end of the document.
Package totpages defines a label TotPages, but it also
makes the register it uses available as a LaTeX counter,
TotPages, which you can also reference via \theTotPages. Of
course, the counter TotPages is asynchronous in the same way that
apge numbers are, but snapshots may safely be taken in the output
routine.
The memoir class defines two counters lastpage and
lastsheet, which are set (after the first run of a document)
to the equivalent of the LastPage label and the
TotalPages labels.
Both count1to and totpages need the support of the everyshi package.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=howmanypp