While there are a relatively decent number of motherboards that have 6 (or more) PCE-E slots, most of them don't work well with 6 GPUs since they are not designed for it. There are allot of posters across the internet that have been unable to get more then 4, sometimes 5 GPUs working on any variety of Z170 motherboards. As long as you can find one of the proven, "limited hassle" 6 GPU motherboards to purchase I would highly recommend sticking with those models.
*I would be very happy to read of first hand experience with any of the above listed motherboards. More variety is always a good option to have.