nicoskaralis:
I'm sure, my LED strip is an SMD 5050 that uses the remote controller bellow and has 4 wires: +12V, R, G, B
So you want to use the strip without that controller, and make your own with transistors and the Arduino.
Then we still need to know total current draw of the strip (the number of RGB LEDs).
Your drawing/simulation/thinking is totally wrong.
One strip section/colour is made with three LEDs of the same colour in series, with one current limiting resistor.
There are three of those per strip section (dotted line), for the three colours.
Remember that these are RGB LEDs (three colours/LEDs inside one 5050 package)
The transistors just act as a switch.
How hard you have to 'push' that switch to stay fully 'on' depends on how much current that switch has to switch.
So again, how many 5050 LEDs do you have on your strip.
Leo..