How many people are on depends on time of day. Maybe arrange with some of your friends to turn up on a server together if you're finding they're generally empty. (Mornings in Europe can be very quiet.)
The ReaNINJAM client displays several of the public servers with who's on, as do:
http://ninbot.com,
http://ninjam.dyndns.org/index-new.php,
http://llcon.drealm.info/peter/llcon and several other sites that I don't remember (sorry!).