One doorbell host can add up to 20 buttons/detectors
One button can be add into unlimited doorbell host
You can buy compatible wireless botton in this link: 
Click here
KR-M521 Home Use Wireless Doorbell with LED lights and Loud Speaker
Technincal Parameters
1. Working Voltage: AC 100-240V
2. Working Current: < 100 mA
3. Wireless Frequency: 433 MHz
4. Wireless Distance: 100 meters (Open area)
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+86-0755-83957680
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