I used to use Redmine at a company I worked for several years ago. There was probably about 20 of us in total. It worked well.
I always liked how Redmine has a tree of projects, rather than just a list.
OpenProject (www.openproject.org) is a fork of Redmine. I have not used it, but I did extensive research on it a couple of years ago while looking for a new project management system for our division. I think it is worth a look.