This can depend on so many factors it's hard to say with any certainty. There have been members that have over 200+ devices on one hub and are still running happily with rules and everything.
A second or third hub could be employed if the area of the devices is large - say for a detached shed or garage or just a large property.
Otherwise maybe adding a second hub by device "type" - Like my setup - I have a C7 for Z-wave devices, a C5 for Zigbee devices, and another C5 for cloud/network apps and devices (Alexa, Flume, Lutron, etc).
Chances are you'll be fine with what you have and thanks to HubMesh add something later on if necessary. Using the "lite" versions of RM whenever possible and limiting 3rd party apps also helps with resource utilization. I prefer to keep my rules off hub by incorporating a companion server running Node-Red..
You can also monitor your hubs free memory and other attributes using @thebearmay's excellent HubInfo driver.