Even with a stock 68000 you can mix 8 channels.
For a 68060 mixing 8 extra channels takes no time.
There are many different ways of mixing though, and some are far more cpu intensive than others. There are various types of interpolation as well as oversampling and so on.
Playing a 32 channel fasttracker mod on an 060 with software mixing (oversampling) & panning @ 44.1khz uses a fair amount of cpu. And this using ptimised mixing code (I have faith that platon42 knows his stuff!).
