Oddly enough I'd never checked before, but as of now there are 1426 accounts. The reddits told me once that about 1 out of 10 visitors there is logged in, so this seems about right.
[1] http://web.archive.org/web/20050414040411/ycombinator.com/sfpannounce.html
lynx -source http://news.ycombinator.com/user?id=$USER_IN_QUESTION | grep '<span class="comhead">' | wc -l