If you put them in series, it will make 21 LEDs x 12V = 252Volts, correct?

If yes, and you are in Euro zone with 220VAC line, then convert 220VAC to DC, and drive them with a single mosfet only with 12F683 pwm output dimmed.

You will get some heat due to voltage difference but "I think" it can be taken away from the mosfet with a small size cooler.