limitations of using multiple RTL-SDR dongles simultaneously

[Olivier SCHMITT]

Hello dear RTL-SDR users,

I whant to use multiples dongles as array for decode wspr.
My problem is that I can not use more than 4 USB dongle simultaneously.

I check with usbmon, each dongle uses 500kbits / s around. So I am far 
from the maximum bandwidth allowed by usb 2.
I tried using a USB switch with external power supply but it does not 
work better.

The default test performed is:

#!/bin/bash
for ((i=0;i<=4;i++))
do
         rtl_test -d $i -s 250000 &
done

First 4  is starting but last 5 doesent. And i whant to use more...

Is this a limitation of the RTL-SDR library or the kernel driver?
Do we have a solution to get around the problem?

Thank you in advance for your valuable feedback.