I think this is largely based on the type of devices you have and the driver software for them. I have a few wifi based devices that if i had allot of them I could see being a problem. Something like Lutron with Pro hub I would speculate would far less impactful. You also don't want to underestimate the impact of user apps on the hub as well. By far the biggest impact on my hub is the Ecobee Suite app. When you look at how much load that one user app puts on the hub there is do doubt of the impact user apps can have over simple automation rules. If anything from the few times I have asked this question it depends on your overall setup and no one rule applies to how many of something the hub can handle.
As a general statement though I would say any Wifi or cloud device like Ecobee Thermostats, wifi Bulbs or speakers, or Roku's will take more cpu cycles then directly attached Zwave/Zigbee devices.
The good thing is that you can see that info now on the logs page with App and device statistics so you can see if you are starting to overwelm the hub in some way.
My suspicion is that if everything was kept local on zwave/zigbee or local network hubs like lutron/Hue the Hub can support allot more then we give it credit for.
My only suggestion would be if you have a bunch of Cloud/wifi devices or apps that will work with Hubitat, put them on there own Hub device once the scaling starts to become an issue. The point that will be a problem is largely dependent on your setup.