I've been working in the career planning field for more than 20 years. During that time I've heard various experts talk about how often people change careers in their lifetimes. These figures usually range from between three and seven times. Often these statistics are attributed to data collected by the US Department of Labor (DOL). The funny thing is the Department of Labor doesn't keep track of that sort of thing. The Bureau of Labor Statistics, part of the DOL and the collector of all sorts of employment-related data as the title implies, "has never attempted to estimate the number of times people change careers in the course of their working lives" (FAQ about National Longitudinal Surveys on the BLS Web Site). As to why the agency hasn't done this, the FAQ goes on to say: "The reason we have not produced such estimates is that no consensus has emerged on what constitutes a career change."
This reasoning makes sense to me since career change, in fact, can be defined in different ways. For some it could mean switching career fields, while others might equate it with getting a job with a different employer. Still others might say they've made a career change when they've advanced within their fields. The BLS, in a press release dated September 2010, released the results of a report that looked at the number of times people changed jobs. This report was limited in that it only considered job changes that occurred between the ages of 18 and 44 and only looked at a small subset of the populationthose born between 1957 and 1964, a segment defined as "young baby boomers." The report showed that those people changed jobs, on average, 11 times (Number of Jobs Held, Labor Market Activity, and Earnings Growth among the Youngest Baby Boomers: Results from a Longitudinal Survey).
Whether that number signifies a change in place of employment or career field, it is a big one. Perhaps you can increase your odds of having career stability by putting more thought into choosing a career or deciding whether to accept a job offer. Better decision making could lead to finding a job or choosing an occupation for which you are well suited thereby increasing the chance that you will stick with it. Of course, even taking great care when choosing a job or career doesn't mean you won't want to or need to make a change. There are other reasons you may decide to leave your job or change your career.