This question was posed to me in a comment earlier this week: how many icons does Office contain?
The answer: it depends on how you're counting.
The most common icon size in Office is 16 pixels high by 16 pixels wide, but we also have 32x32 and a limited set of 24x24 icons in use as well. Additionally, we have a number of graphics used to render the UI which are not icons at all but are either bitmaps (the old way) or PNG files (the new way) which end up compressed into the Office DLLs.
Within each icon, we usually have multiple color depths: 32-bit including alpha channel for the icons you see in the Ribbon, sometimes 256-color versions as well, and definitely 16-color versions for icons that have been around for a while. We also have certain black-and-white variants of icons for use in high contrast mode.
So back to the original question: how many?
The visual nature of the Ribbon, in which every feature has an icon, has meant creating a lot of new icons.
If you just count 16x16 icons (where one icon = one unique shape rendered into any number of color depths within a single .ico file), we have well over 3000.
If you counted each color variant as a separate icon, that number would be much higher.
We're edging ever closer to 1000 32x32 icons for use in the Ribbon as the days go on...
Suffice it to say, there are a lot of pixels being pushed around.