AFAIK one limitation is forging of the large pressure vessels, mainly the reactor itself and the steam generators. Those take a long time. Of course you can do multiple ones in parallel, but then the limit is that there's about 25 heavy forge presses on the planet, of which only a subset are doing nuclear work. So in total the capacity at the moment is maybe 50 large pressure vessels per year worldwide. If one plant requires one reactor and 4 steam generators that would then be 10 plants per year.

Of course it's possible to increase the worldwide heavy forging capacity, but again, something that takes time.

Then there are many reactor designs that don't use water as a coolant or moderator; those are obviously not restricted by heavy forge capacity. But those reactors are mostly on the drawing board and not available off the shelf.