I'm sorry to pepper you with questions but as my post count indicates I am a complete newbie.
No problem! I'm somewhat of a newbie myself, I work in post production in the film industry and recently delved into the industrial automation world for a rather large home automation project (Click PLC based with AdvancedHMI on a wall-mounted touch screen).
If you check the previous link to the click expansion modules, you can get cards with 16 outputs (or 16 inputs) on one card. There are also combo cards that split it so you get 8 inputs and 8 outputs on one card. The Click supports up to 8 expansion cards, so that's 128 outputs on one PLC. The amazing thing with PLC's however, is a simple serial connection between them (eg over rs485 or rs232), the systems can now address each others inputs and outputs and a myriad of other things. So if you max out one plc with 128 outputs and need more, You can drop in another Click PLC with it's own expansion cards quite easily and have your hmi plugged into one plc be able to address and control the other plc as well, all on the same system.
Keep in mind PLC's were designed with this exact kind of application (high reliability, unpredictable scalability needs, etc) in mind. So things like adding another plc to the system is very easy and is done often.
EDIT: I just realized where you got the 8 output number, you're looking at the modules with built in high voltage relays. That can be convenient for controlling small numbers of things, but how it's done 99% of the time is using the typical low voltage solid state outputs (like on the normal output cards) to control external relays, either mechanical or solid state. This way you don't have to home run nearly a hundred different 120v lines all the way to your plc to be switched and then back again. You would usually run control wiring from the plc's low voltage output to whatever needs to be switched and have relays locally mounted there. This is how I'm controlling all the lighting in my house, I have a bank of solid state relays in the attic that intercepts all the lighting wiring locally, and a simple run of cat6 ethernet running from the relays back to the plc in the utility closet. (Cat6 turned out to be a great choice for a home project anyway, it's 8 full conductors (so control of 7 relays all sharing a common) and getting 200 feet for less than 20 dollars is incredibly easy on ebay. the 23 or 24awg is more than enough to carry the 50ma~ current required to switch a relay.)
Another thing to keep in mind is mechanical relays (like in the Click's AC output module) do have a lifespan due to moving parts and will eventually fail. So if you're using nothing but cards this means one input means replacing the whole card, however if you have external relays distributed around your building you can only replace what needs replacing. Better yet, if you use solid state relays, they will have a near-infinite lifetime assuming you use them at or below the rated current and do not let them get above their recommended max temp. If you want some suggestions on some cheap and reliable SSR's let me know.
Regarding Click and AdvancedHMI - With AdvancedHMI you use the ModbusRTU driver to talk to the click, which is just modbus over serial. The driver was pretty wonky a few months ago but I tested it as much as I could for Archie with a couple other Click users and he pretty quickly got it working perfectly. It's now plug and play and doing things like just turning things on and off is very easy. I'm currently using it with some on and off switches, temperature display and controls, and house power consumption monitoring displays. It was pretty easy to do now that the RTU driver is working well. No altering of the program was necessary which is good because I can't code for the life of me.
Hope that helps!
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