Physiologically the liver can produce and store thousands of stones since there are about 50,000 bile ducts in the liver. So as strange as it seems on a intuitive level, it is possible to flush out thousands of stones. As to the stone size issue, bile ducts expand so large stones can form.
The best evidence I know of is first hand experience. I thought the folks on this site were mistaken after believing Dr. Michael Murray's assessment that liver/gallbladder flushes were just juice and oil. By simply doing a few flushes, the variation of what comes out will show you that it is not just oil/juice and the improvements in dietary fat tolerance and other improvements will convince the most serious doubters.