Our controller comes in 8, 16, and 24 zones, can be expanded up to 64 zones.
If all of your wires are together in 1 place, all you need is a 24 zone controller. If they're in 2 different places, you will need to install 2x16 zone controllers. You can purchase them at Metro irrigation supply in Austin.
You can do it yourself. An irrigation controller basically has a number of valve wires and 1 common wire. The wires are not labeled, but you can tell what they are by looking at where they're connected. It's just a matter of replacing the wires. The main thing is to identify the common wire.
If you're not comfortable, you can contact any local irrigation contractor and they'll install it for you.