What is a Trappist beer and why are there so few?

Trappist isn't a style, it's a seal. Only beers brewed inside a monastery, by Trappist monks or under their supervision, with profits going to the monastery and charity, can carry it.

Abbey beers like Leffe don't count: they have the name, not the seal.

Why are there so few? Because there are fewer and fewer monks, and no monks means no seal. Achel lost it in 2021 because it had none left. Today there are fewer than 10 in the world, most of them in Belgium.

Double, Triple, Quad: monasteries name them three ways. With crosses, like the chalk X's that marked the barrels. With colors, like Chimay's red, white and blue. And with numbers, like Rochefort's 6, 8 and 10.

If you see one, treat yourself.

Source: Wikipedia: Trappist beer