20GW is nowhere near 20 plants at full capacity, it's 20 reactors, and is probably more of an average.
Reactors range from 400MWe to 1600MWe, and plants generally have 2 to 6 reactors with 2 outliers at respectively 7 (Kashiwazaki-Kariwa in Japan) and 8 (Bruce in Canada, although it's only running 6 reactors right now). The 10 biggest nuclear plants all have more than 4GW installed capacity.
FWIW, France has a total of 20 active plants grouping 57 reactors. The installed capacity is almost 62GWe.