The uno consumes very very little power, usb supplies 500ma i've not done the calculations but you should be able to run at least a couple hundred of them (and a few hundred more if you remove everything except the atmega chip) ..
The linear regulator is a huge power waste, bypass that and supply 5v direct it would use even less power http://forum.arduino.cc/index.php?topic=83612.0
From the spec sheet:
• Power Consumption at 1MHz, 1.8V, 25°C
– Active Mode: 0.2mA
– Power-down Mode: 0.1μA
– Power-save Mode: 0.75μA (Including 32kHz RTC)
So that's uh..
A lot of chips you could run from a 5v USB supply, but with the regulator, the chip for USB communication add it all up, its still a lot of uno's.... off a 5v usb supply.