The number of people needed for a trial depends on the size of the effect.

I think we have a warped idea of that, because of the practice of looking for a barely "statistically significant" effect from a substance that isn't really understood.

If you have something that just eradicates a disease immediately, because you really understand what you are doing, you don't need very many tests to confirm it works.

In principle you could put people into multiple trials and gain somewhat additional throughput. Google implemented putting users into multiple different experiments (paper by Tang et al) and that made a huge difference.