See the statistics generated with the database dumps:
http://stats.wikimedia.org/EN/TablesDatabaseWikiLinks.htm

You even got charts:
http://stats.wikimedia.org/EN/PlotsPngDatabaseWikiLinks.htm

bloodice.spelled wrote:

Thanks, but I know about them. My intention is to include this number in a page
(in my user space), and to calculate dinamicly the link per article. Is it
possible or it is much work? I wonder what is the mechanism to count interwikis
trough database - does it includes scan of all pages, or there is some clever way?

robchur wrote:

The only mechanism we've got at the moment would be to count all rows in the
langlinks table. That would drive people spare if implemented as a magic word,
so someone is going to have to come up with a clever solution.

Add it to Special:statistics, maybe?

robchur wrote:

That's not a mechanism, that's a further output location. We need an effective means of
calculating the number, first, that won't cause our database servers to whimper and die.

webmaster wrote:

How about a cached number?
The data wouldn't be 'live and accurate', but then again... what is these days with a dozen or so squids up-front?

I don't think a single number would really be meaningful here, even if it was added. There'd be no distinguishing between wide coverage (many pages marked) and deep coverage (many languages linked).

Going ahead and WONTFIXing this. Not a massive use case, and as Brion mentioned above, it's not a very useful number anyway.