Yes, you should be able to control many “Switch” type of devices (e.g. X10 Appliance Modules or Wall Switches) via one Arduino. You will need to create a custom Virtual Switch DeviceType. You could start with one of my Virtual “GarageDoor” DeviceTypes, and simply change the icons and limit them to only two states (on and off). There are probably other examples of this as well floating around.
You will definitely need a custom SmartApp to link the Virtual Switch devices, to your Arduino DeviceType code. Take a look at my Mux/Demux SmartApp as an example. You will probably need to pass to the Arduino, via the DeviceType code, a numeric index to represent which X10 module you want to turn on/off. Similar to how I handle the Open/Close commands for my two distinct Garage Door relays.
Here is a link to my post about my garage door Arduino code (Arduino code, ST DeviceType, SmartApp, and Virtual Contact Sensor + Virtual Garage Door switch/button) Arduino SmartShield Garage Controller
Hope this helps!