It's a good question. It's unclear what the answer is. Dell specifications state a maximum of 32 GB for the system, but do not state a per-slot maximum. Crucial states system maximum of 32 GB, but not a per-slot maximum. However, Crucial's guaranteed-compatible RAM list does not include a 32 GB module, perhaps indicating that a single slot can accept a maximum of 16 GB. Kingston states "Maximum Memory Per Slot 16GB". Based on that information, I would conclude that you can not use more than 16 GB in a single slot. If you want to try a 32 GB module, get it from a vendor that will allow for return and refund if it doesn't work.  
One of my memory slots died (will not boot up if there is anything in that slot, even a known-good SODIMM). I am wondering if the computer will recognize and use a single 32 GB SODIMM in one of the slots or if I'm stuck with only 16 GB.
XPS 15 9560 Can I use a single 32 GB SODIMM?