It depends on the type of sensor. Some sensors do have jumpers in the inside that is used to set the address. Other sensors have the code build into the sensor electronics.
By setting the jumpers to be exactly the same for more than one sensor the alarm will pick up all those sensors as the same sensor on the programmed zone as they contain the same address.
Sensors without these jumpers will mean that only one sensor can be set per zone.
Please note: Sensors without jumpers are usually sensors with a more advanced (much longer), thus more secure, address code.
We prefer selling the sensors without jumpers (improved wireless security) and an alarm system with more zones rather than sensors with jumpers and alarms with less zones.