I would buy a 16 zone. You might wish you did later. I should have the first time.
I doubled upsome valves onto one zone with good success, but only valves closer to the controller. The DC voltage did not reach the longer runs with enough current to reliably open the valves. It is dependent on the gauge and length of wire. I have a large yard.
Trial and error will help you determine what will work for you, but it can be done.
