I was able to have two spare boulders (relatively easily), and I suspect three might be possible, but the boulders at the top are rather intertwined to the point that it's difficult to get them out without wasting that third one anyway.
Four and above I think are even theoretically not possible.
Rest in peace, Kym. I hardly knew ya. Rest in peace, Marinus. A bright star, you were ahead of me on my own tracks of thought. I miss you.