AFAIK Debian isn't very diligent about tagging packages with "implemented-in", but even so:
dpkg -l |grep ^ii -c #Installed packages
3608
# packages tagged as being in c++, and installed:
aptitude search '?tag(implemented-in::c++)' \
|grep ^i -c
930
I'm sure there's lots of c++ in the other 75% (or they depend on a runtime/compiler/library written in c++) -- but at any rate - one in four packages is nothing to sneeze at.
On a different desktop right now, so can't check -- but that'll probably lower the count considerably (eg: on a different server I get 226 for "c/c++" and 19 for just "c++" ("c\+\+)").