Strips that use the 5050 RGB LEDs are fairly easy to calculate.
They put 3 5050's in series with a resistor to work at 12volts. Like most LEDs, the 5050s use 20 milliamps. So for every cut-able segment in the strip, they will need 20 milliamps per color, so 60 milliamps if using White (all colors).
So, if you are using 20 segments, that is a max current of 20 x 60 or 1200 milliamps (1.2 amps). I mention the total because the COMMON wire used must be able to handle the TOTAL current, not just R or G or B.
So, for 12V strips, count your cut-able segments, multiply by 60, then move the decimal back 3 places. That is the amps needed.