UTorrent does not behave well when its download or download temp folder is shared with other apps (especially the OS pagefile). Some folks have gone higher, but some aspect of the code design leads to instability when too many torrents are loaded. For best stability, try not to exceed 3000 torrents. This configuration was used to help reseed TTD. With the ultimate setup, I had about 900 simultaneous downloads transferring for a combined speed around 35MBytes/sec.
