It MIGHT be possible to make a guesstimate for CloD using the number of downloads of the TF patch. So far over 2,300 have downloaded the torrent file I created shortly after the patch was released. Add to this the number of direct downloads through the link TF posted, plus the mirrors and torrent file made available by the Russian community and one might get a pretty accurate number for CloD sales, by now most CloD owners have probably installed TF's patch. Unfortunately I don't have the figures for the direct downloads or the Russian torrent file(s), but perhaps somebody else does.
As for the original IL2, FB, PF, IL2:1946 etc I haven't a clue but the worldwide figure must run into the millions, if not tens of millions. Add to this the figures for all the addons such as BoE, Barbarossa etc and the total volume of IL2 related sales must be truly staggering for such a "niche" product line.