At least in 22.11 (and in 23.05) it defaults to 1. It's been said that 20.11 defaulted to 0. There's some consensus it would be more helpful to have it default to quantity ordered or zero instead. I found bug 6903 for reference.
I have been thinking about it some more, maybe instead of ordered it should be quantity ordered minus quantity received? (pending/unreceived items) And 1 for standing and subscription orders?