I'd probably say "one person less", myself--not exactly grammatical, but at least sensible and less awkward.

I would have probably said "one person fewer". But you probably knew that already.

"the number of people in the loop had shrunk by one"

Are you seriously suggesting that you or anyone else would naturally utter the sentence "the number of people in the loop had shrunk by one"? That sort of stretches the imagination right there. But even if that were a possible utterance, it doesn't help with how you'd phrase that idea if you get as far as "there was..." and aren't sure how to finish the sentence. I guess you could always call it a train wreck and restart the sentence, but I feel like requiring such a strategy is needlessly prescriptive.

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Say whether the tens digit of this number is even or odd: 14
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