Technically, each root hub can support 127 device on a five tier level (as per the spec). The limitation is due to the number of different addresses that
can be assigned to the device. The address field is a 7 bit field and hence can have 127 individual addresses. See section 220.127.116.11 of the USB spec for more details on this.
In reality, the host need to keep up with the devices. The host will only configure a device (the device will enumerate) if the bandwidth that it needs is available, and the power requirements are within the spec limitation. Please see chapter 7 and 11 of the USB spec for this. Of course if any of this is violated the device will fail to enumerate. In reality rarely is the case when 127 devices are attached to the same Hub port.