If you are talking about simulator there is a hardcoded restriction on 4 MC motors + 1 pusher. If you have more than 4 MC motors than control signals would be applied incorrectly and model will not fly.
This PR supposed to deal with issue. Bit it is not merged. And not sure it works.
My quick fix is here. But it is a hack not a solution. And it works only for 1 push motor.
As for real RC models with 8 MC motors:
In my case I have MC motors working over UAVCAN and other motors/servos via PWM. So I have to use 2 mixers. And there are some issues with having more than 8 motors on a UAVCAN due to mixer groups size restrictions. But that’s a different story.
As for PWM motors most of pixhawks has at most 8 of PWM signals on MAIN and AUX. So you still have to split your mixer file on main and aux parts.