The bottleneck will probably be the total USB throughput available in the USB controllers in your system.
With a much weaker (and older) system than yours, I can burn 4 DVDs at the same time to one internal and three external drives at 8x from just one harddrive.
Is what you want to do possible? I don't see why not.
What is the limit? Who knows.
Problems likely to occur? Maxed out throughput in your USB controller(s).
For connecting external USB burners, you may have to get an additional USB controller.
Limiting the burn speed to 8x or 12x might be the difference between success and failure, and it won't take that much longer than burning at 16x or faster. Plus the risk of poorly burned DVDs will decrease.
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