Well, it depends on whether I'm local or not. Local, I think I could handle as much as the ATC has to give me in the transmission....after all, I probably know exactly what he is going to say before they say it.
In a place that I haven't been to before, I would figure that I'd reliably handle well over 4, so long as it wasn't spoken too fast.....so enough that my pencil can keep up! Particularly so with non-standard reporting points. (I'll have sussed out the standard reporting points in advance, but for non-standard ones I'll be trying to spell it out in full rather than shorthand, to aid me finding it on the chart later).
One instructor I had insisted that I always write down my clearances etc, and only attempt the readback once the clearance was written down. And the readback came not from what I remembered, but instead came from what I had written down. That way I know that what I have written down is correct and can be refered to as often as necessary. It was one of the best pieces of advice I've ever got, and doesn't really take much time
dp