How many crates can the plane drop in one flight? I have seen two but there are times where I haven't seen a single crate get dropped (probably because it was dropped very far away).
Mid last year a YouTuber gathered community feedback as well as tested on his own to try and figure out how Air-Drops work. In his testing he found that double crate drops would happen infrequently, but in testing never saw more than two drops from the same plane. Though the testing was done before the 1.0 launch (and crate contents have changed since testing), it is unlikely that the number of crates spawned has changed.
PS: Anecdotally I have never witnessed nor heard of another player witnessing more than 2 crates from the same plane, so it seems impossible or at the very least extremely unlikely bordering on impossible.