Does anybody know how many zigbee devices (ST sensors) a mains powered zigbee switch can be the parent of? Another words how many devices can they repeat for?
I know the limit of zigbee devices is much higher if you route them through mains powered repeaters first before going to the hub. I’m sure it’s depending on the manufacturer, the ones I am mostly using are the OSRAM plugs.
I think that depends on the device and what the manufacturer has designed the device to do. Some can support 32, while some will only do 8. It depends. @JDRoberts, do you have any handy resources that can be referenced?
I have heard some users advise “6-8 endpoint devices per repeater” as a rule of thumb for a very stable, resilient Zigbee mesh network.
How do you control the number though. I can’t see anyway of forcing a device to connect to one repeater over another.
With ST, who knows the logic; but I’m sure there are professional tools out there to manage a mesh. I typically have 3 or 4 or more mains powered zigbee devices per room, so when I add a new battery powered device, I always do it in the room it will end up, or I rejoin it in the room I’m moving it to.
You can’t. Except by unplugging all of the devices that you don’t want the child to select as a parent at the time the child is joined to the network.
It’s quite a complex algorithm as to which device a child will choose as a parent and it’s not intended to have human intervention. 
But if you must, for whatever reason, leave only the one parent You want on power When you add the child. However, if it anytime the hub loses power and then restarts, all the children will reevaluate potential parents so they will probably choose one of the ones you didn’t want them to. So then you have to go through the whole network build from scratch again. Very annoying, but it is what it is.
That’s more than most inexpensive zigbee home automation devices can handle, The typical number of children a parent will support in the price range that most smartthings user select is 3 to 5.
But it’s totally up to each manufacturer. They can have it support more if they want to, but then that takes more resources and adds to the cost of the device, so again, it’s typically 3 to 5 for a non-hub.
The network will let you know if there are no available parents because you won’t be able to add the next device. 
This is typically the way I found to do it. Just wondered if there was a more scientific way 
I kinda wish there was too, but that method seems to work so far. I’ve got 235 zigbee devices now, with about a dozen more planned (replacing zwave stuff), and so far so good in terms of performance and reliability.