The coil of a relay has a built-in resistance (all that thin wire really does add up in resistance). You will probably see a rating like "12V 100mA" which, if you do the Ohm's Law calculation, tells you that the resistance of the coil is about 120 ohms.
So, the answer to your question is what you wrote yourself: it only draw as much current as is necessary to itself
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The QuadRAM shield: add 512 kilobytes of external RAM to your Arduino Mega/Mega2560