Even if there are multiple undiscovered planets, there would be at most one planet nine, plus maybe a planet ten, eleven, etc.
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What would happen if we discover an object that deserves the term "planet" at 180 AU, and then five years later discover another at 120 AU? Which one is "nine"?
Both. The one at 180 AU would be considered the 9th known planet from the Sun, until the one at 120 AU was discovered.