Depends on the age of the motherboard.
To clarify;
When Switched-Mode Power Supply's were first made for personal computers, for the main power cable they used two of them.
This was the AT power supply.
http://www.playtool.com/pages/psuconnectors/connectors.html#oldpcThe next version of the SMPS used a single main power cable, the ATX power supply.
http://www.playtool.com/pages/psuconnectors/connectors.html#atxmain20As personal computers increased in Processor power, and graphics power, it was found that more power to the motherboard was needed. The ATX main power cable was increased from 20-pin to 24-pin, to handle the extra load,
http://www.playtool.com/pages/psuconnectors/connectors.html#atxmain24A lot of motherboards also receive extra power through a 4-pin ATX +12 Volt power cable,
http://www.playtool.com/pages/psuconnectors/connectors.html#atx12v4This power cable is used for power to the Processor.
Power Supply's (SMPS) have increased in power over the years to keep up with the demand for power.
They have also increased the number, and type of power cables used.
Some now have an extra 6-pin PCI Express power cable for added power to a graphics card, or two 6-pin PCI Express power cables.
Some SMPS's have an 8-pin PCI Express power cable for added power to a graphics card.
http://www.playtool.com/pages/psuconnectors/connectors.html#pciexpress8If an SMPS has an 8-pin EPS +12 Volt power cable, it is used for added power to the motherboard, for the Processor.
http://www.playtool.com/pages/psuconnectors/connectors.html#eps8If your concern is how much power can your motherboard handle, know this;
A computer ONLY uses the power it needs, and NO more.
If you had a 10,000 Watt power supply, and the computer only needs 100 Watts, the computer only uses 100 Watts. (10,000 Watts is an exaggeration)
Do you know the motherboard's name, and Model Number?
State in a Comment, and I should be able to tell you what power cables it needs, and give examples of an SMPS that will work.
SMPS,
http://en.wikipedia.org/wiki/Switched-mode_power_supply
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