Hello all,
Great Question ~~~ and I wish that I knew the answer only sooner !!!
Well I found out the hard way after climbing (82) Zigbee devices that the total in the system is currently (64) !!! Then Xfinity told me I had to back that amount down - back to (64) !!!
You would think the Salesman and Techs would have told me that from Jump Street - but as usual, they did not and then off I went on my own, buying and pairing away ~ Oh My !!!
This total does NOT include their Cameras. However keep in mind, it does include Motion, Smoke, Flood, Door/Window, Siren Repeaters, Glass Break Detectors, KepyPads, Keyfobs, Lighting/Appliance Controllers, Door Locks, etc...
So there I sat (with especially) lots of Lighting/Appliance Controllers in a brwon bag - unpaired and useless !!! 
However !!! There is a God !!! It's called "Wink Hub" !!! My CentraLite Lighting/Appliance Controllers paired right up with the Wink Hub and the Android App works great with my Smartphone !!!
I have asked Xfinity and Wink to talk to each other and I hope soon to see Wink become a 3rd party vender and integrated their apps together !!!
Not the total 100% solution, but a good 90% in the meantime until Xfinity/Comcast can maybe come up with a Touchscreen or system that can handle more than (64) Zigbee devices in total !!!
For now... I pray !!! (LOL!!!) 
https://www.facebook.com/groups/WinkUsersGroup/
https://www.facebook.com/groups/WinkUserTradingandSalePost/?ref=suggested_groups
Best regards,
Jim 