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.).