You can try debian statistics -- Maxima is included in the standard distro
actually the most common platform is probably windows, now that there
is a good self installing windows distribution (reference
http://www.ma.utexas.edu/maxima.html ). I should put a counter on my
page for downloads, but the thing is these things just get
mirrored--something I would of course encourage not prohibit, so it is
probably not worth the effort.