that is a seriously neat looking project!! Hell, I barely fill a 4 port hub so building one of these monsters is not on my schedule!
I use up to 13 depending on whether the school flash drives are in.
EVDO USB720 modem 2x 500ma (Y cable)
portable USB HD 500ma
logitech iFeel mouse 500 ma
USB WIFI 500ma
printers 2x 100ma
xbox gamepad 400ma
occasional flash drives 2x?
comes to maybe 3-4 amps.
the cool factor is whats nice .. "Resistance is futile, we will assimilate your USB devices!"
plus 31 is a nice number