1. The reason why only one zone can run at a time is that the AC power adapter usually only provides 800mA. One valve can consume 300mA and some people have a master valve or pump which also consumes 300mA. If 2 zones run with a master valve, the total current would be 900mA which is unsafe. However, if you do not have master valve or pump, you can connect 2 zones to one slot in the controller.
2.We would like to help on the zone planning but we are not experts in this area. However, how many zones you need depends on the sprinkler types and also the water pressure. If a sprinkler can cover a larger area, you probably need less sprinklers. I would suggest you consult an irrigation profession for optimal zone planning.
3. As each zone is unique, with different plants, sunlight and soil conditions, we recommend to place one sensor in each zone for best watering results. Consider adding sensors for the zones that you care most. However, in a future version of the app, we will allow one sensor to monitor up to 3 zones but the user needs to make sure the zones have similar environmental conditions.