The short answer is none. 
The smartthings platform just isn’t stable at this time. The platform as a whole is going through a major transition: new app, new hub, new cloud platform. And they are specifically updating their Z wave implementation to be able to support the S2 security framework, and that has introduced a couple of glitches recently.
Hub Firmware Release Notes - 25.26
Smartthings has told us multiple times that their typical customer has 15 or fewer devices and never uses any custom code. So if you stay in that range, you’ll probably at least fit into what they test for.
I know that support on multiple occasions has told people that they are likely to have problems if they are running more than 30 zwave devices. I have no idea where they are getting that number from, and it hasn’t been posted officially.
There is a known problem if you try to add more than around 15 Z wave locks to one hub, but that seems to be specific to that device class. And it’s considered a bug, they are supposed to be looking at it.
There have been some community members who had over 150 zwave devices And they ran into all the usual platform and stability issues: sometimes things would work great for a while, then things which had been working great would stop working, then there would be a known bug that would get fixed, and repeat. Sometimes the workarounds or the fixes for known bugs were complicated by the fact that they had so many devices.
@johnconstantelo , @bamarayne , @anon36505037 , @ogiewon have all had large networks and may have more to add.
But really there’s just no telling.