CrossRoads:
How many motors did you say?
Right now is 6 motors, but might reach 10 eventually.
groundFungus:
It seems we have a confusion of terminology. When you say pin I think of a GPIO pin, but I guess that you mean the 5V supply (Vcc). Not usual to hear the Vcc output referred to as a pin. If Vcc comes from USB there is about 500mA available from the Vcc output. If powered through the power plug or Vin, the 5V regulator can supply about 1 Watt (current depends on how much voltage the regulator must drop). Powering 3 A4988s from Vcc should be no problem.
Ya sorry about my terminology, i understand what you mean now. With each a4988 logic supply current needing 5-8mA (datasheet page 4), and since i am powering my arduino through the 12V jack, 10 should be doable.
MorganS:
Smart driver chips such as the A4988 will reduce power when not stepping. The continuous power you measured is about 70% of the peak power when running.
I should've probably mentioned that i'm microstepping, and according to page 17 in the above datasheet i listed, it should be 100%. So that's why i'm confused when i set the potentiometer to 0.4V (and the math to figure out the current is [0.4 / (8 * 0.1)] = 500 mA), why i don't see close to half the total amperage (250mA) when i measure the current on one coil when the motor is powered but not turning.