Luckily for us, Uncle Sam counts up stuff like this. The Bureau of Labor Statistics (known to most, simply, as BLS) reports, in its survey of Current Employee Statistics, that health insurance companies employ 440,400 workers as of spring 2010. And surprise, surprise! The industry has grown quite a bit in recent years, up from 327,700 employees in 2000.
Need more information? Newspaper archives are one of the best online research tools.