I want to connect multiple DSLR cameras to one desktop computer using SmartShooter software. My question is, what does a USB hub need to support to meet my needs?
There are two things that matter for USB hubs, latency and throughput. Latency is a measure of how long it takes for information to pass through the device and throughput (or bandwidth) is how much data can move through the device in a given time.
Your exact needs will depend on what you are doing, but if you want precise timing from the computer, then you will need a device with low latency. If you are going to be sending large files, you also need a high amount of throughput.
A key thing to realize about USB hubs is that they are limited to the characteristics of the port they are plugged in on. A better approach, if possible, would be to get USB expansion cards that can add additional USB ports to your system. This would allow for the software to communicate with each camera directly, at the same time, instead of having to go to one and then the next and then the next.
You will get both better latency and more throughput if you have additional USB hosts on your system rather than trying to split a single host between multiple devices.
It is also worth noting that there is nothing particular special about this as it relates to photography. The same advice that works for photography will work for any other usage of USB ports, so someplace like SuperUser may be able to provide even more detailed assistance.