Cal, the ATX specifications recommend that the +5VSB line provide 2 amps of current. The 10 mA mentioned in the chart has to be a typo. That's barely enough current to light an LED.
The only sure way to know how much current can be drawn is to check the specifications label on the power supply.
My ChiefMax 450W supply does indeed supply 2 amps at the +5VSB line.