a NSLU2 can serve much more.
Streaming by itself is only limited by the available bandwidth.
Now when you do transcoding, again the bandwidth (we’re transcoding to RIFF/WAVE) is a limit.
Only the short time periods when multiple clients browse songs, playlists etc, the CPU usage spikes. But even on a NSLU2 those periods are reasonable small.
So unless you’re talking hundreds of clients, a regular PC will do fine.