If you visit Apple's
PR Library, you can find the quarterly sales results posted. Each one has a PDF you can download that includes a summary of Macs sold. The most recent one,
from April 19, 2006, includes a PDF ("Data Summary" at the top of the press release) that only breaks down the Macs sold into "desktop" and "portable" computers. There's no way to distinguish between iMacs, G5 towers, Xserves and Mac minis in the desktop category. Older press releases like that (for example,
the April, 2005 release) break down the desktops into iMacs and PowerMacs, but the iMac group includes the Mac minis and eMacs so it's still not possible to figure out exactly how many Mac minis were sold.
So, unless Apple releases the numbers separately, I'm not sure it's possible to answer your question. 😟
-Doug