Which xiangqi pieces can never deliver checkmate by themselves, no matter how many you had?
These answers restate the rules implemented by our open-source engine, which follows the WXF laws for movement, checkmate and both repetition rulings: a side that forces a repetition by perpetual check or perpetual chase loses rather than drawing. The chase half has not been validated against the official casebook. Where an answer refers to a specific position, that position lives on the linked reference page, where the build verifies it before publishing.
Direct answer
The advisor and the elephant, no matter how many you have. Checkmate requires directly attacking the enemy general, and both of these pieces are permanently confined to their own half of the board, the advisor inside its own palace and the elephant behind the river. A hundred advisors could not deliver check on their own, because not one of them could ever reach a square near the enemy general.
Why quantity does not fix it
With every other piece type, more copies eventually mean more attacking power somewhere on the board. Advisors and elephants are the exception: their movement rules are a hard boundary, not a matter of degree, so no amount of material of just those two types changes what they can reach.
Every other piece can do it with only its own general's help: a chariot, a horse, and even a single soldier that has not yet reached the last rank each force mate against a bare general. The cannon sits in between. It delivers mate constantly in real games, but alone against a bare general it has nothing to jump and cannot force mate at all.
What they contribute instead
Their value is entirely defensive and entirely indirect. Your own advisor never leaves your palace, so it can never guard a piece near the enemy general, but it can serve at home as the screen your cannon fires over, which is how cannon and advisor beat a bare general. The defender's own advisors and elephants, meanwhile, often help the attacker by blocking their general's escape points. Either way the final blow always has to come from a chariot, cannon, horse or soldier; your general helps under the flying-general rule by taking a file away from the enemy general, but never gives check itself.
Related questions
Can an advisor or elephant assist in a mate even though it can't deliver one?
Yes, very often. The defender's own advisors and elephants occupy palace points their general would otherwise use to escape, and your own advisor can be the screen for your cannon from inside your palace.
Is the general in the same category?
In one way, yes: the general never gives check itself, because moving it to face the enemy general is illegal. Unlike the advisor and elephant, though, it often takes part in a mate by denying the enemy general the file it stands on.
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