There's the 5V pin on the power header, and then there are the 20 output pins.
You can use the 5V pin, for maybe 200-300mA total if the onboard 5V regulator is being powered from 12V. The regulator has to dissipate (12V - 5V) x current drawn as heat, and it's just a little chip without much heatsinking.
(12V - 5V) x 0.2A = 1.4W for example.
.5A = 3.5W, the regulator will either go into thermal shutdown, or just plain fail. How many motors did you say?
Don't use output pins - 40mA is absolute max, and a burst of current higher than that can damage the pin and the 328P chip.