Yep. In 2017 it was 10-12 per month (about 50-50 captain/FO) and one month it even got up to 18. 4-6 sounds low, but there are definitely retirements/separations included in the attrition numbers. Not all the attrition is to the airlines, but from the posts on the message board, at least half is--even to regionals and ULCCs.

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