That kind of depends. The biggest factor is nitrogen fertilizer. Without the Haber-Bosch process, there is just simply not enough nutrients in all available soi

That kind of depends. The biggest factor is nitrogen fertilizer. Without the Haber-Bosch process, there is just simply not enough nutrients in all available soil to grow nearly enough crops. But that process was invented in 1900 and doesn't require any remotely advanced technology. The ancient Romans could have pulled it off with the technology at the time, though probably at much smaller scale. I guess it depends on whether you want to consider chemistry a technology. The chemistry to turn atmospheric nitrogen into ammonia has obviously always been there, we just didn't discover it until recently. If you told an ancient alchemist how to combine the ingredients under pressure, they could produce fertilizer from air, iron, and acid. Is that knowledge a type of technology?