user39711: How many TIP122 transistors can I connect to the Arduino digital pins? The Atmega328p datasheet states that the pins can source a total of 200 mA, while other documents state 40 mA per pin. 
In the following circuit layout, I connected six TIP122 transistors with 2.2  kΩ  base resistors. I used an LM338 regulator to power the Arduino IC and other functions.


user42539: I would use a N-channel MOSFET instead of the TIPwhatever.
AOD514 from Digikey has really low Rds, somewhere around 0.01 ohm, and can be driven from an Arduino pin, say thru a  150 ohm resistor to protect against the gate input capacitance of the MOSFET. A 10K pulldown resistor on the gate will hold the MOSFET off until the Arduino sketch starts up and controls the pin.
AOD514 is surface mount, but tab and pins are pretty good size and can be manually soldered.
AOD508 and AOD510 are even lower Rds, but cost a bit more at Digikey.

39711: <p>How many TIP122 transistors can I connect to the Arduino digital pins? The Atmega328p datasheet states that the pins can source a total of 200 mA, while other documents state 40 mA per pin. </p>

<p>In the following circuit layout, I connected six TIP122 transistors with 2.2  k&#8486;  base resistors. I used an LM338 regulator to power the Arduino IC and other functions.</p>

<p><a href="https://i.stack.imgur.com/iveUk.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/iveUk.png" alt="circuit schematic"></a></p>
 42539: <p>I would use a N-channel MOSFET instead of the TIPwhatever.
AOD514 from Digikey has really low Rds, somewhere around 0.01 ohm, and can be driven from an Arduino pin, say thru a  150 ohm resistor to protect against the gate input capacitance of the MOSFET. A 10K pulldown resistor on the gate will hold the MOSFET off until the Arduino sketch starts up and controls the pin.
AOD514 is surface mount, but tab and pins are pretty good size and can be manually soldered.
AOD508 and AOD510 are even lower Rds, but cost a bit more at Digikey.</p>
