What amp breaker for a 220 volt 8000 watt sauna?

A sauna heating element is a resistive load. The equation that you are looking for is Amps = Watts/Volts = 8000/220 = 36 amps.

A #8 copper conductor with an insulation rating of 90 degrees C is rated at 55 amps. This will be fed with 40 amp two pole breaker.