What are the 75 most-subscribed health insurers in the U.S.?

It's hard to answer partly because enrollment varies so much by region or market.

The American Medical Association used to publish "Competition in Health Insurance" which included competitive analysis by state, though not nationally. It no longer publishes this book. The last version was in 2002 with 2000 data.

The Government Accountability Office also has done some discovery, but just on the small group market. See its report at 

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Also see this fascinating report on the managed-care industry. It doesn't cover enrollment data but does cover profit/loss and other data points on major players: 

www.cms.hhs.gov/reports/hcimu/hcimu_03242003.pdf

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Some state governments also report on conditions in their state.