check this thread to learn more than your ever wanted to know about HUBs: https://bitcointalk.org/index.php?topic=253749.0

the table tells you how many erupters work in each one.

the 127 hard limit on USB is per controller. Depends on how many USB Controllers your MB has. PCI cards can be bought with their own controllers to add capacity.

I'm running 24 on 3 10-port HUBs with no problems.