Looking at my WinXP "task manager" display, I see that "System Cache" is currently 465MB. Now the OS uses that to hold many different things, but also recently read disk data. The question becomes not how many files, but how much space they total together.
If I wanted to read the same 200MB of file data again and again, I (likely) would not see the disk light flash because all the data would fit and still be present in the cache memory. If I needed to repeatedly process 500MB of file data, I likely would find other things to do while the disk light flashed for many minutes, _each_ time, because all the data could not fit in cache and _must_ be re-read each time.