How can the person you're replying to answer the question about how many people should be able to block it? If we look at the consequences (as I am inclined to do), then it's going to depend on the specifics. If the airport is not going to bring a tangible benefit, then it doesn't matter if it's one person or ten thousand blocking it. On the other hand (putting it very roughly), if it brings great benefit, then we can estimate those benefits and mitigate the harms to those forced to sell by compensating them well for it. If the compensation to those displaced outweighs the benefit, then there's your number regarding how many should be able to block it.

As for the trolley dilemma, it's a great thought experiment precisely because people's answers and their reasons for their answers differ. I'm not sure therefore of its relevance here. Presumably the person you're responding to can just be consistent in their response to both the question about airports and the trolley dilemma.

> "What you're arguing is the classic of the trolley dilemma [1], the individual vs the many. Should you harm 1 person to save 1 million."

Of course if that's genuinely the situation, it's hard to argue that the needs of the many shouldn't be paramount. But in the case of airport expansion, it's rarely that simple!

A bigger airport isn't necessarily a public good. If it's close to urban areas it's often a net detriment to the public good, inflicting more noise and pollution on millions of people. The drive to expand really comes from the airport operator's desire to increase flight numbers and profits.