This is discussed briefly in the wireless range FAQ. It’s less about how many than it is about distance.
It’s also important to note that it is up to each manufacturer to decide how many children each zigbee repeater can support.
The smartthings hub can support 32 directly connected zigbee end devices. Others zigbee repeaters tend to support around five, but some might only be three where others might be seven or even more. It does require some memory and processing power to handle each child, so there is a cost factor even if it’s not much.
All of which is to say if you try to put 33 Sengled element lightbulbs ( which don’t repeat it all) in one room with the ST hub, one of them will keep falling off Until you add an additional zigbee repeater in that range.
Speaking just for myself, I try to put two repeaters of each protocol in each room. That should be way more than enough, and it gives me a very strong mesh.
At a minimum, with typical US construction, you will need one about one every 40 feet. If you live in a house with brick or adobe walls, you might need more.
Zigbee is an Omnidirectional signal, which means you can count that 40 feet as the radius of a sphere around the repeater device. 
Also note that zigbee home automation, (ZHA), which is the profile that smartthings Uses, is limited to 15 hops into the hub and 15 out. As a practical matter, that will not affect most residential buildings, but again if you have brick or adobe walls it might.
{as mentioned previously, I was a network engineer and had worked with Zigbee and Z wave devices before ever buying SmartThings for my own home.}