I am running 11 clients on this setup:
cpu: i7 2600K Sandy Bridge
mb: Asus P8Z68-V
RAM: Kingston 32 GB DD3
VGA: Gainward GTX 1060 6B
LCD: 2x 24" Benq

Right now when I am doing nothing just idling in low level zone, my CPU is at 55% and RAM at 24GB. As soon as I am porting just 5 toons to Dalaran, the game crashes, because 32 GB RAM is the limiting factor here. I have to port 1 by 1 and RAM settles at 31-32 GB. All toons except 1 leader for each group has vga settings to 1, leaders got 6.

This way I am getting constant 60 fps on which ever toon I am active on. I highly doubt, that 32 GB RAM will be enough for 15 toons, if you want to do something more than idling in town. As soon as I am playing doungeon or pvp, RAM usage goes for 2-3 GB per wow.exe instance. I had to lower the graphic settings even more after the prepatch 8.01. because they changed the game and it suddenly much worse to even start the game. Where as on monday 11 clients started 1s after each other, now it is 5s, and logging (even choosing which toon to play) tooks sometimes half a minute! So now I have clients set to /console renderscale 0.4 on all except leading toons, and starting the game is significantly faster. I dont mind having toons on lower settings, because 90% of time I have active windows on leading toons.