There is a limit to how many songs can be displayed in a folder at one time. This limit is adjustable in your settings, but the larger you make it the less memory you'll have left for buffering mp3 data => more disk spinups => less battery life.
It really makes sense to organize music in folders, e.g., by genre/artist/album, etc. Then keep the max files down to a reasonable number. You can always use a playlist if you want to group all 750 songs together for some reason.