What I observed in late May was that the line for the, roughly, half-hourly busses from the left-front of the Sorrento Circumvesuviana station had a line with more than the 55 or so people who would fit on a bus. I presume that those who failed to pack onto the "next" bus after they got in line simply continued standing in line until they could get on a bus. I don't think many of them abandoned their excursion plans for the day.
Note that other posters have mentioned a local tour company that runs slightly more expensive busses on similar routes. That might be an affordable solution. But it seems unwise to assume that you will get a seat (!) on the side of the bus with a view towards the water.
I think that more people go to Positano than go to Amalfi, so perhaps it is easier to get a bus from Positano to Amalfi than it is to get a bus from Sorrento to Positano. Did you read the recent rant here about this bus service? OTOH, there are plenty of people who were grinned and bore it!
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