Hi, I’m switching to Hubitat. Have many different devices (over a hundred) and a large home, 3 levels, over 9k sf. Will one c8 hubs be enough to cover my home, or do I need multiple, such as 1 on each floor? Thanks.
Both Zigbee and Z-Wave make use of "repeaters" (or "routers," though colloquially both are often called the former) to extend the range of your network. So, all of your devices do not need to be in range of the hub. Z-Wave allows up to four "hops" between the hub and your device, and Zigbee allows for...more, often cited as 10 or so (this may depend on your specific devices or repeaters). This difference is somewhat balanced out that Z-Wave tends to be able to reach larger in a single hop (lower frequency, plus a generally less crowded piece of the spectrum). But given the strict limit on hops, Z-Wave is often cited as a concern.
So, let's look into that. If your house is 9000 sq ft over three levels, let's assume 3000 sq ft per level. If that's a perfect square, then it would be 55x55 ft. Theoretical range for Z-Wave maxes out at over 300 ft in open air, but indoors you'll more often see 30 ft recommended as the maximum distance for an indoor "hop." With a centrally located hub, that still gives you full coverage on each floor, possibly even without a repeater on the same floor as the hub and with as few as maybe a couple on each end the others, depending on ceiling height and resulting distance. Of course, that's a theoretical minimum; more is almost always better. 
If you have switches or plugs, etc., in each room, all of those are repeaters and will likely help you (you don't need dedicated "repeater" devices, though they do exist, and their ability to be positioned almost anywhere might be helpful in some cases).
Of course, chances are, your house is more rectangular than that. And perhaps there is lots of concrete or other things that are more of an RF obstacle that wood, drywall, etc., than some of these "averages" assume. But the point is, it's theoretically possible with some planning. There are still some reasons you might want or need two hubs -- for example, Z-Wave is limited to 232 devices, and certain types of Zigbee devices may not play well with others on the same network (e.g., certain bulbs). Whether you want to start that way is up to you — and a little bit of mat, or really the reality of RF, which is hard to predcit ahead of time. Luckily, Hubitat introduced a feature a couple years ago to make devices easy to "share" between hubs, so this is still more to manage but not as difficult as it used to be.
Good luck either way!
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A single hub is likely a good start.
An option to help with future expansion could be to get two - one for devices, one for applications, but people that have multiple hubs have them setup in different ways based on their needs and preferences.
Some background:
I started off with a single hub, and then added a lot of devices (real and virtual) to provide close to 100% automation in my home. It has been my experience that some community apps and drivers will require more processing power, and because of this, I ended-up with an environment which 1 hub for “approved” Z-Wave and Zigbee devices, one for non-standard Z-Wave, Zigbee and Wifi/LAN devices (support by community drivers) and one for all my dashboards, rules and most apps. Hubitat has a feature called “Hub Mesh” which allows me to link all the devices from the “Device” hubs to the “Rules/Dashboards” one.
Because I started with all devices on the same hub, I had to modify all my rules and dashboards once I moved my devices to the other hubs. However, now that they are on a separate hub, I can easily replace them or move them to other hubs and just move the Hub Mesh link to point to the new hub. This makes maintenance much simpler, and is why I would recommend this as a setup to help with future expansion.
Thank you folks very much with the detailed responses. Very helpful!! One more question, any issues with mixing hub versions? Say 7 and 8?
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No; features that you'd need to make them both work together, like Hub Mesh (or some community solutions) would work on any model, though there are some hardware and feature differences between models.
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To add to @bertabcd1234’s response, one of the advantages of the C-8 hub is its use of external antennas which help it reach longer distances before needing to go through a repeater.
If you decide to go with the approach to have one of your hubs not use its radios, then the C-7 hub would fulfill that purpose as well as the C-8.
Thanks. So just for the virtue of better coverage, is it worth alone to purchase C8, to take advantage of the sale going on now? Or should i just stick with C7 i have. I see C8 also have upgraded radio versions.
The amount of coverage you are talking about .. I would start with a C-8.
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