How to increase the resoltuion of a 20 khz pwm Wave (Atmega 328p)?
If you mean by "resolution" that you need to have more than 8 bits, you will need to use your other timer/counter (the ATmega328 has one with 16bits). To get 12 bits of resolution you would need to do the following:


Use TC1
Set a lower prescaler to enable faster counting, roughly a factor of 8 should be fine
Set an upper value of e.g. 4096 (=2^12 bit), this is the value where the counter shall reset
Set a threshold/OCR value of a value between 0 and 4096, this is your output value