For 900 mA, R should be 1.2 ohms rated at 2W. To control it via an Arduino, where the 2k2 resistor is shown, rather than going directly to the supply voltage, you want to it be controlled by the Arduino and it will need actually to be about 68 ohms and since each current driver will draw about 60 mA, to pull 12 of those to 5V (720 mA) you will need a BD140 with a 150 ohm base resistor.
If you think that is a lot of transistors (and the BD139s and BD140 must be mounted on a quite large heatsink), you are right!
If you do not want to control it with an Arduino, then the BD140 is not necessary, you just use 180 ohm resistors where the 2k2 is shown above, to your 12-15V supply.