I currently run 15 on:

I7 3930k @ 4.4ghz
2x GTX 970s stock
32g ram
WoW from an SSD

All 15 run at 1920x1080 with minimum graphics on the 14 followers and medium graphics on the main. They're set to run at 20fps in the background, 60fps in foreground.

There is only one place in the open world I've experienced issues: The outskirts of Shattrath. Just the raw number of NPCs my machine is rendering tanks my fps down to 10-15. In Ashran, I have my 'follow' key setview on the followers to zoom all the way in and look at the ground. I get ~20fps on everyone but it's still totally playable. That's pretty good considering my machine is rendering 200+ characters.

My graphics cards are never maxed, but the CPU does hit the roof in the places I mentioned. Maybe when the next generation of processors becomes available I'll upgrade and see about performance.