Sizing an airline's cabin crew workforce

Estimate how many flight attendants a major airline employs.

Roughly 15,000 to 25,000 for a large US carrier. Start from fleet size (say $800$ aircraft), crew required per aircraft (about $4$ on a narrow-body, $8$–$12$ on a wide-body, so average around $5$), and hours flown per day (about $12$) versus hours one attendant can work (about $80$ flight hours a month, roughly $3$ hours a day averaged). That needs about $4$ crews per aircraft-seat position, so $800\times5\times4\approx16{,}000$, plus reserves for sickness and training.