Not only ramping up/down but all pulse creating has to be handled in the ISR. But it needs less time in the ISR if the timing of the pulses is constant ( constant speed ) and not varying from step to step.
It's all not a hard limit - it depends. The more steppers are moving at the same time, the more load to the ISR. And the more are accelerating or decelerating at the same time, the load increases even more. The speed of the steppers also matters. At the end you have to try what is possible.
Which Arduino are you planning to use (not all are supported so far)?
You don't need to create the line, it's already there. You only have to change the '6' to '9'