It's not the computer as much as it is the hard drive array. Test it with something like the BlackMagic Disk Speed Test and it'll tell you. 1080p30 HD, say DNxHD 185 or ProRes HQ, requires about 90MB/s to play one stream smoothly. So if your disks test out to be 500+ MB/s, you can get five streams of video to play back smoothly.
Although other factors like quality playback settings in Premiere also affect this, but not by too much. Might get another stream to play. This isn't a processor thing, it's a speed of the disks plus connection type thing, mainly.