You will only be able to run up to 128 devices on your USB bus. Each of the 4 (or however may backside/frontside USB ports you have) ports are all connected to one controller on your motherboard. The extra ports just come from an internal hub (which is also on the board). So, you can have up to 128 devices hooked up to your USB controller (128 total). However, because USB cannot provide unlimited amounts of power, most of the hubs providing these 124 (minus the 4 you already have) extra ports would have to be powered to prevent your USB bus from being overloaded. So, if you'd like to have 128 external hard drives talking to you computer through one 480Mb/s bus (actually slower because of USBs master/slave design, which is unlike IEEE1394/firewire) then do so.
I pulled it out of a dark (but very knowledgeable) place. No, not Canada. (for any VGCats viewers)