How many PWM output channels does the Atmega4809 really have?
I am not sure where does the number 8 come from, but I can see total of 10 possible PWM outputs in the datasheet.

The RTC has no easy &amp; direct way how to create PWM output.

On the other hand TCA timer can be run in split 8bit mode. Citing from the megaAVR family datasheet, page 193:


  Split Mode Overview To double the number of timers and PWM channels in
  the TCA, a Split mode is provided. In this Split mode, the 16-bit
  timer/counter acts as two separate 8-bit timers, which each have three
  compare channels for PWM generation. The Split mode will only work
  with single-slope down-count. Split mode does not support event action
  controlled operation.


You can enable this mode by setting bit 0 in CTRLD register. The meaning of CTRLB register changes then and you can enable all 6 outputs there. So, if it is acceptable to have 8bit only PWM with common clock source for all 6 channels, you can use this mode.

Atmega4809 datasheet shows, in page 8, which pins can be outputs of TCA0 mapped to.

TCB timers add 4 PWM channels, one for each counter.

If you really need more PWM channels, it is maybe possible to get one or few more (in a bit hacky way) through the custom logic unit. Use suitable source of periodic events (maybe the RTC?) to flip an output of CCL as needed.