Hi,
I'm not sure if Adobe ever releases specific numbers. It can also be a bit of an odd statistic for a few reasons. Photoshop gets bundled with other Adobe products, as well as being sold individually, so a popular bundle might inflate Ps numbers. It targets a profession market, so sales volume compared to software targeted at the much larger consumer market are dissimilar. Hardware changes can drive significant software changes (such as the Mac change to Intel processors). General economic cycles will impact sales, and don't directly correlate to popularity.
I'm sure there are other factors I haven't listed, too. As for figures, I don't have any to provide, sorry.
regards,
steve