I need to drive 20 motors each of rating 12 V DC and 4 W power. My purpose is just ON/OFF various combination of motors continuously making use of the arduino. I don't want to involve too many batteries and drivers and make the module bulky. Want to keep it as compact as possible. Please help me out. Thanks in advance. 
If your setup just requires on/off, you probably can use latching multiplex chips like the 74hc259 driving logic level MOSFETs.
Hi, this is my first post here.
I'd like to do the same thing but with 3 or 4 DC motors (4.5V each) only.
I managed to drive only 2 of them without any problem by following these instructions:
but when I try to add a third motor, to a second L293D H-Bridge this time, I fail.
I suppose that in order to use 3-4 DC motors I'll need an external power supply.
Any suggestions?
Thank you in advance.
To run most DC motors and servo an external supply is required. USB cannot supply
enough current for all but the tiniest of motors.