But the controller in the USB-Hubs also count as a "device" some of the bigger ones have two controllers in it.
That is NOT a controller......
It is not capable of initiating a transfer on its own, as such at the most it is a PERIPHERAL DEVICE.
And as such it is subtracted from the total count of 127.
However for practicable reasons I have yet to see a 'fully loaded' USB system.
A guy I used to hang out with did do a full on 127 device chain, back when USB1.1 first came out, and it worked. Bogged the system something fierce, though. With the faster throughput on 2.0 and 3.0, I think it would probably be better. However, it's probably safer and EASIER to just keep the numbers down and use multiple hosts. A usb controller isn't exactly expensive. Even this cheap ass acer I'm using has two of 'em.
Ideally you get something like a PCIe card with several controllers (NOT USB ports), then build the chain off that.
For SBC's there is a big issue in that you need data-sheets and a circuit diagram to see how it is implemented, I think on the first revisions of the PI it is badly done.
One absolute killer is where they use a bloody chip for the Ethernet with extra USB ports on, some of these chips do not even have a TCP/IP buffer!!! so once you get one of these shitty little cutters on your USB chain, you have a chip with multiple Ethernet ports open, with no buffers... sucking the bandwidth out of your USB chain.
Even worse are the 'pack of lies' controllers which say USB2.0 compatible, but when you pull the data-sheet they are actually USB1.0 , but they are classed as 'compatible' because the 2.0 infrastructure is capable of communicating with the 1.0 device!!!
Only issue is... if you read the USB spec... a USB chain with a 1.0 device on it, defaults the WHOLE chain to 1.0 spec speeds.... (yep they do have some 'hubs' with translators, but it depends on the chipsets used.)
It is like a massive Microsoft nightmare, where even accomplishing something a simple as plugging a couple of devices into your computer becomes a major research task.