Since you can copy selected midis into named user playlists as described in great detail in the step by step instructions on the pages previously quoted in my book it should follow there is no need to copy them one by one from the default playlist since any user list can be created automatically at the copy point.
The default playlist is it, the figures quoted are maximum figures. However there is no reason not to save midis as technics files so the theoretical maximum number of midis on an SD card would be 20,799 songs. Practically using high quality retail midis the sequencer holds 6 to 8 songs, so the practical number would be around 15,000 songs.