Spark-8 supports upto 8 zones (similar Spark-16 supports 16 zones). For each zone, you can customized the sprinkler type, soil type, plant type, etc. For your case, you have 6 zones, with 3 or more sprinkler heads in each zone. So Spark-8 should be enough.
The whisperer sensor is optimal. It helps to get your soil moisture data more accurate than estimation. However, without the sensor, the Spark can work as well. It's recommended to place one sensor in each zone, however, you can share the data to upto 3 zones if these zones have similar settings (plant, soil, sprinkler, etc.).
How is each sensor associated with each irrigation zone?
Is it possible to tell the system in which zone each of the sensors is located so that it has a record of the sprinklers or set of sprinklers that affect it?
is there a price difference?
Currently we only sell EU spark 16 and we will launch spark 8 in the future.