shoulda said most have facility in 5 or 6 keys..for standards/jazz players:
F G Bb C Eb, then Ab for some..then a few more advanced players can handle A and/or D, so there are 5-7 keys that would benefit from a transposer.
Those that work in guitar-based idioms learn to be comfy in G,A,C,D,E more than others.
same good opportunity for transposer if other keys desired.

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Miami Mo