Electronic – What will be the current of a 24V ATX power supply

According to the EPS12V standard section 6.1.2 Processor Power Connector, the requirement is that the connector uses two different power rails (12 V1 for pins 5 and 6 and 12 V2 for pins 7 and 8) if the 240 VA limiting is in place. So if nothing else is connected to those two power rails, you should b able to pull 20 A through the connector if it's divided correctly among the two "rails".

You'll most likely have to limit the current yourself. It's under that condition that the voltage is specified. Going higher than 200\$\mu\$A will probably cause the voltage to sag below 2.4V, which is the minimum for a high level in TTL. (I've also seen 2.7V as the minimum, I guess it depends on the TTL subfamily.)

Note that the same table says that "signal type = +5V TTL compatible", and that a low level is specified as < 0.4V. That output level is 0.4V less than the maximum TTL input level for a logic 0. And the 2.4V is 0.4V higher than the minimum for a high input level. This gives a 0.4V noise margin.

If you want to control a MOSFET with the PWR_OK signal, like OP, you'll need a logic level MOSFET, which draws enough current at a low \$V_{GS}\$. The BSG103 may be a good choice; it has an \$I_D\$ of 750mA at a \$V_{GS}\$ as low as 1.5V.

edit
On second thought the BSH103 may be too good. It has a \$V_{GSth}\$ of 0.4V, which means that worst case you'll have a drain current of 1mA with PWR_OK low. Even FETs with a \$V_{GSth}\$ of 1V typical indicate 0.4V as a minimum value. Can be fixed by using a resistor divider to lower the output voltage from PWR_OK. A 15k\$\Omega\$ + 25k\$\Omega\$ gives you a minimum gate voltage of 1.5V, while the current is maximum 125\$\mu\$A.