Maximum zones

My current sprinkler system has 17 zones (valves).
Is it possible do use the 16 zone system of yours and put the wires for two valves in one zone?

Usually, there is no problem. If the valves in question draw more current than average, you may get a false ‘zone fault’ and will need to add a relay to control those zones. I believe that a zone output can safely supply 300 mA.

If you also have a master valve or pump start relay that draws a lot of current, the total load might exceed the capacity of the Rachio power module, so you may also need an external transformer to power the paralleled valves.