It should hold around a 1000 songs of relatively low bit rate mp3. You didn’t specify what sort of mp3s you have. If they are high bit rate CBR, then 250 might be reasonable.
Also, if you are adding files from both your PC and your Mac, then the issue could be MTP versus MSC. If you left Fuze’s USB mode at the default setting of Auto, then it’s probably using MTP mode when connected to your PC. The Mac will use MSC mode. Files added under a given mode are only visible when connected under that mode. In other words, files added from your PC will not be visible to the Mac and vice-versa. I would suggest formating the Fuze, setting the USB mode to MSC, and then readding your files. As long as you are always in MSC mode, your PC and Mac will both be able to see the same files on the Fuze.
