Here’s what Wiki has to say:
San Diego International is the busiest single-runway large hub commercial service airport in the United States, with approximately 600 departures and arrivals carrying 40,000 passengers each day, and over 18 million passengers a year. The busiest single-runway airport in the world is London Gatwick Airport.
Confirmed by, again, Wiki:
Gatwick has two runways but the northern runway 08L/26R is a non-instrument runway and is only used when 08R/26L is temporarily non-operational because of maintenance or an accident. The runways cannot be used at the same time because of insufficient separation between them. It can take up to 15 minutes to change over from one runway to the other.