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.
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public PGP key
http://pastebin.com/dqZqgyVE