MC has been tested with libraries with over 1 million files and is said to still perform pretty well. There are a good number of people with libraries ranging from 150,000 to 350,000 files that seem to have no issues.
The size of the media files themselves doesn't really matter, as long as your hard drive(s) can hold them. It's really the number of files and the metadata about the files that influence JRiver MC's speed in showing them to you on screen.
Brian.
