Irrlicht Engine

Is niko still involved in the development of Irrlicht, or is that left to the other developers while he works on stuff for Ambiera, e.g. CopperCube and IrrFuscator?

Niko is still involved, e.g. for the packaging of the stable releases. He's also doing some tests and from time to time fixes compilation bugs. Regarding development of new features he's focused on the Ambiera tools, though.

So how many active developers are in the project?

Depends mostly on time. Every developer in the author section of the Irrlicht webpage can add stuff to svn when he finds time to do so. Right now many of us (including myself) seem to be rather busy, so the last months were a little slower. But I suppose that will change again, otherwise we will probably have to think about adding more developers again at some time. Also a lot of additions are coded from people here on the forum, so a lot of activity is going on outside the core-team.