According to Autoblog, the limit is set at 420km/h to protect the tyres from disintegrating. Willi Netuschil, head of Bugatti’s engineering department, said the top speed can be increased to about 458km/h if a customer wishes to. He also mentioned that customers is unlikely able to achieve such speeds as a much greater braking distance is needed at these levels.
Oh, and just for fun, the average Chiron customers is said to have around 42 cars, 1.7 jets and 1.4 yachts. Sounds reasonable to us!