The 400 GW capacity is derived from around 440 nuclear power reactors operating in 32 countries, providing about 10% of the world's electricity in 2022.

https://world-nuclear.org/information-library/current-and-future-generation/plans-for-new-reactors-worldwide.aspx 

Nuclear energy, with around 413 gigawatts (GW) of capacity contributes to both goals by avoiding 1.5 gigatonnes (Gt) of global emissions and 180 billion cubic metres (bcm) of global gas demand a year.

https://www.iea.org/energy-system/electricity/nuclear-power 

Leading countries include the US, with 100 GW of capacity, France with 60GW, China with 50GW and Russia with 30GW. Japan’s nuclear capacity is presently around 45GW.

https://thundersaidenergy.com/downloads/nuclear-capacity-forecasts-construction-times-operating-lives/    

The largest reactors under construction as of May 2023 were situated in the UK, with a gross electricity generation capacity of 1,720 megawatts. Meanwhile, the country with the largest capacity of nuclear reactors under construction worldwide was China, where reactors with a total capacity of almost 22 gigawatts were being built at the time.

https://www.statista.com/statistics/267158/number-of-nuclear-reactors-in-operation-by-country/