The 20mA per colour is the specification of the individual LED.
With a 12V supply it is likely that there are some LEDs in series and so the currents are not cumulative. That is if you have 3 LEDs in series with 20mA flowing through each the total current is 20mA not 20 * 3 = 60mA.
It is only when the LEDs are in parallel that the currents add.
So it is safe to assume that this product takes 2A per strip.
k but in the strip 3 red leds are in series, 3 green leds are in series, 3 blue leds are in series and each series combination connected in parallel. so that we will have 150 series combinations(each color) in parallel.
2A current must be devided in to each parallel branch then what may be the current in each branch ?
On my commonanodestrip it's all 3 LED in series (per Strip)
every kathode (each color) has a corresponding resistor. (150/330/330).
If you dont divide the strips, every 3-led-strip is parallel.
So assume 60mA per element
I have a similar strip. The red leds have a fwd voltage of 2, and use the 330 ohm resistors per 3 leds. Blue and green are 3V and each use a 150 ohms resistor. Use http://led.linear1.org/led.wiz to tinker with this. Now, about the amperage I made same calc as you did to arrive at 60mA*50 = 3A. Either the supply is beyond max or it isn't lighting the LEDs properly. In any case, it's a shame they didn't make the strings twice as long, in which case they could have left out the resistor on the red leds. Now, how do drive these with my arduino ?