When you think about it, London does seem to have a lot more airports than it should. In descending order in terms of number of passengers per year, there's Heathrow, Gatwick, Stansted, Luton, City and Southend, which is ridiculous, as YouTube vlogger and musician Jay Foreman explains in episode three of his Unfinished London series.
Believe me when I say it's a bit exciting (if you're a sad infrastructure geek like me) that Foreman's actually got around to making a third episode - it's been three years since episode two, and the first one came out in 2009. They're witty walkthroughs of the ways London's stop-start, never-finishes-what-it-starts approach to urban planning has made the city what it is today. Episode one deals with the Northern Heights plan, which would have connected the stub of the Northern line with Mill Hill East on it with Edgware, and likely have brought with it thousands of new homes in what is now, still, a semi-rural area; episode two is about the Ringways, which would have seen London surrounded by four ring roads, of which the M25 is the main survivor (and which was abandoned halfway through, thus why the "south circular" isn't).
Episode three, though, is about the capital's airports, which are mostly built on what were aerodromes built in the inter-war period by wealthy plane nuts indulging their hobby, who in turn saw a money-making opportunity in accepting passengers. Yet the first airport in the city, Croydon Airport, couldn't survive after the Second World War as its runways weren't long enough for new, larger planes, and neither could most of the other aerodromes or airports, which found themselves surrounded by urban sprawl. We have Heathrow where it is because it was near the edge of the city, with enough room to be the city's main airport - and Gatwick is even further out because it was the best candidate of the nearby RAF airfields to be London's backup. And, as they in turn became constrained by planning issues, Stansted (an RAF airfield) and Luton (a small regional airport meant to serve the Home Counties) were commandeered to serve ever-growing London. City was part of Canary Wharf's regeneration, and Southend is the most-recent, becoming London Southend as part of a rebranding exercise. That brings us to six.
This is only civilian airports doing international flights, though, because "airport" is a fuzzy definition. There are nearly 20 further airfields and aerodromes within Greater London and just outside it, as well as RAF Northolt - and then there's the fact that some airports that aren't meant to serve London are, nevertheless, as close as some of those that are (like Lydd Airport in Kent), while some that say they're meant to serve London (looking at you, "London" Oxford Airport) probably don't in practice. If you include all these extra types of airport then, well, it's probably impossible to judge who has the most - a city like Los Angeles, or Moscow, will easily match London on it.
Worth noting, too, that Foreman points out that a Thames Estuary airport was first proposed back in the 1970s, before London did what it's always done - take an existing airfield instead, and expand it. History has a funny way of repeating itself.
Here's hoping episode three, part two doesn't take another three years to arrive.