What is and how to increase the maximum number of usb devices?

It's by standard as explained here: https://en.wikipedia.org/wiki/USB_device#System_design

You can't have more than 127 devices (hub inclusive) connected to a single host controller.

Now: the host controller+3 hub (1st layer)+18 hub (2nd layer) = 22 devices that DON'T appear as disk devices.

127 minus these 22 USB devices = 105 devices that can be managed by the kernel as disks.

127 devices is a hard limit of the USB protocol (see here: http://www.beyondlogic.org/usbnutshell/usb3.shtml#USBProtocols) because the ADDR address field is seven bit long.

So by software there is no way to increase it. Maybe you can try with more than a host controller interface, or change topology by reducing the number of hubs (thus increasing the number of disks seen by the system).