the nice part about hubs is they lie.. if you unplug the it from the mains and turn the plug over it should have a sticker on it with a section labled
"output --- 5v  3a"
or something similar
USB hubs are a funny thing as alot of people are finding out theres very few that do what they atually say they do
iv got a cheap ass 1amp one that wont even power a single erupter
 

so looks like your limit is just 2 erupters and even thats going to be unstable as its already got 1 HW error