The math is rather easy, it is however questionable what you define as one of the largest nuclear power plants. Most power plants have multiple units, if you take the power per unit you more or less end up with 20 years.

Capacity of large nuclear power plant unit: ~1 GWe [1] Total world electricity consumption is 20,900 TWh/year [2], of which 40% is generated by coal power plants. A total power of ~1000 GWe, so you'll need 1000 nuclear power plants to replace all coal-fired plants by nuclear power plants.

[1] https://en.wikipedia.org/wiki/List_of_nuclear_power_stations [2] https://en.wikipedia.org/wiki/Electric_energy_consumption