There would be some TM who would have cardinality 'n' and There would be some TM which don't have cardinality 'n' so It's non-trivial for sure so we can first tell that its undecidable.
Now if it's semidecidable ? then Go to Rice's Theorem . it would tell you that containing Exactly 'n' strings is a non-monotone property for RE sets so yes it not even Semidecidable.