limitations of using multiple RTL-SDR dongles simultaneously

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.