How much current do I need for a 5050 RGB strip?
The manufacturer or supplier should specify current per length of strip.

As a "rule of thumb" most small LEDs are rated at 20 mA and groups of 3 are connected in series plus a series resistor to 12V. So approximately power dissipation for N LEDs =
Power = V x I = 12V x 20 mA x N/3 = 80 x N mW.
And I = 20 mA x N/3  

For N = 300
Power = 80 x 300 mW = 24 Watts
&amp; I = 20 mA x N/3 = 20 x 100 mA = 2A.  

The two answers differ.
I think this one is correct :-).
I'll check the ratings of some 5050 LEDs but I think the other answer may not have allowed for the 3 LEDs per package.