Well better to have less number of plants with higher output. As it will reduce the fixed costs, resulting in more cheaper electricity from one plant.
As time goes by, new tech is being rolled out, through which more electricity can be generated with same usage of fuel.
Best option nowadays are the 3rd generation of the nuclear plants, which can be more efficient with less number of plants.
" A generation III reactor is a development of any of the generation II nuclear reactor designs incorporating evolutionary improvements in design developed during the lifetime of the generation II reactor designs. These include improved fuel technology, superior thermal efficiency, passive safety systems and standardized design for reduced maintenance and capital costs. "
Generation III reactor - Wikipedia, the free encyclopedia