Tested several rtl dongles with linux 3.9.1 on Debian Jessie (testing)and got disappointing results
2 dongles simultaneously working ok
if using 3 or more than 3rd dongle not working (1st and 2nd working ok)
in Debian Wheezy i did manage to run 5 dongles simultaneously (kernel 3.2)
i suppose developers should pay attention to than fact.