Bottom line: 47% of those covered by job-related health insurance have only 1 choice of plan; 33% have 2 choices; 19% have 3 or more choices.
Here is a slide from Kaiser Family Foundation showing percent of firms offering insurance, who offer, 1, 2 or 3+ choices of plan (89% of firms offering cover offer 1 plan). Here is another slide showing the proportion of covered (insured) workers covered by firms offering 1, 2 or 3+ plans (47% of workers covered by workplace policies had 1 choice, or no choice; 33% had two choices; and the balance, 19% had 3 or more choices.
Here is the full report, The Annual Employer Health Benefits survey, done by Kaiser Family Foundation and the Hospital Research and Educational Trust. Here is a list of the exhibits and tables....lots of stuff here.