When you're burning different compilations to different burners, with one or more harddrives as the source, then it's difficult to predict where the bottleneck will be.
As an example I can burn large files or ISO images onto 3-4 burners simultaneously (at 8x) with 2 or three of them being externally connected by USB, with all files residing on just a single harddrive.
This is on an older P4 2.4GHz system with a 3-year old harddrive.
Using the exact same system but a 5-year old harddrive as source, it's impossible to burn more than one compilation at a time, or the harddrive will thrash and almost completely stall the burns.
Newer systems should be better able to handle simultaneous burns, but just exactly how many and at what speed will depend a lot on the specifics of your setup.
Limiting burn speed to 8x will help (also helps reduce likelihood of bad burns) or at least no faster than 12x.
Using more than one harddrive as source will also help.
Using an SSD as source should help tremendously.
You'll only really know how many simultaneous burns you can achieve by trying it on your own system.