15-20 might be a little much, especially if all those stream are coming off the same hard drive. Are these compress rips, tv shows, music, etc ?
If they are bluray I think the limit was something like 4 streams from one drive and then stuff started to buffer and lag.
At 20 streams your 1GB NIC will likely hits it throughput limit also.
Going to need a little more info on what you are trying to do.
20 streams of small music files should not be a problem, larger and higher bitrate stuff might/probably will be.