I have more then 8 wired sensors, can I also use two Retrifit Alarm Kits with one Ring Alarm? Or are 8 wired Sensors the Maximum one Ring Alarm can operate?
Hey @Hamburger! Our welcome to Retrofit article states “you can wire up to 8 zones with one Retrofit Alarm Kit. If you have more than 8 zones, you can add additional Retrofit Alarm Kit devices to the same Ring Alarm.” More information about the Retrofit Kit can be found through the links in our Community post about the Alarm Retrofit Kit. If you are unable to find the exact solution, check back here to see if other neighbors have shared a similar experience with their Retrofit Kit throughout the Community. 
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There is a good explanation from Ring to clear the confusion between zones and sensors
_ "What are wired contact sensor zones? How do they differ from the wireless contact sensors that come with my Ring Alarm? _
Typical alarm systems often wire together multiple window and door contact sensors in one area of the home into a single zone , like a string of holiday lights. When any individual wired contact sensor within the zone is opened, the entire zone is considered to be open, and if the system is armed, it will trigger the alarm…"
So a “zone” can for example be “living room” with several sensors/contacts. I guess the question was: With a retrofit kit, can I wire several contacts to a single zone or must each sensor be wired to a unique zone, for a maximum of 8 sensors for each retrofit kit?
Neighbor PHD-2 answered he had grouped several sensors in one zone by wiring them in serie
A bit more technical explanation would envolve explaining the difference between Normally Open (NO) and Normally Closed (NC) panels and why you need to wire the sensors in series and not in parallel