Personally I have had a total of 8 jobs so far and most of those were in my teenage and college years. My jobs: Janitor, electrician helper, library assistant, computer lab monitor, electrician helper, computer lab monitor, UPS auditor, engineer. All of those jobs were 2 years or less except my current job as an engineer. So I've already had 8 jobs but most of them were when I was age 18-25. I've been in my current job for over 13 years. Some of the jobs I've had lasted 3 months or less. I'm not sure if I'm supposed to count those.
Number of jobs in Career
BLS.gov has a National Longitudinal Survey (NLS) In the NLS site FAQ they say:
"These younger baby boomers held an average of 11 jobs from ages 18 to 44. (In this report, a job is defined as an uninterrupted period of work with a particular employer.) On average, men held 11.4 jobs and women held 10.7 jobs.
From ages 18 to 44, some of these younger baby boomers held more jobs than average and others held fewer jobs. Twenty-five percent held 15 jobs or more, while 12 percent held zero to four jobs. For additional statistics on the number of jobs held, see the tables at: www.bls.gov/nls/nlsy79r23jobsbyedu.pdf"
If you look at the table you see there is some variation between age, sex, education level and ethnicity. However just about every demographic group has an mean in the range of 8-12 jobs.
More jobs in younger years
Little more discussion on the NLS has some more information. They say:
"Individuals born from 1957 to 1964 held an average of 11 jobs from age 18 to age 44. These baby boomers held an average of 4.4 jobs while ages 18 to 22. The average number of jobs held fell to 3.2 while ages 23 to 27 and to 2.6 while ages 28 to 32. These individuals held an average of 2.4 jobs while ages 33 to 38 and 2.0 jobs while ages 39 to 44. Jobs that span more than one age group were counted once in each age group, so the over-all average number of jobs held from age 18 to age 44 is less than the sum of the number of jobs across the individual age groups...."
And about job lengths they say: "Although job duration tends to be longer the older a worker is when start- ing the job, these baby boomers continued to have large numbers of short- duration jobs even at middle age. Among jobs started by 39- to 44-year- olds, 33 percent ended in less than a year, and 68 percent ended in fewer than 5 years."
Length of Current Jobs Held by Population
Another look at job length is to look at now long the population has been on their current job right now.
The time you are employed at a job is the 'tenure'. The BLS tracks tenure of current employees. Currently from the BLS report on tenure the median tenure for employees is about 4.4 years. (full report in PDF) There are variations in tenure across job, demographics, industry, etc. 30% of workers had 10 years or more and 19% were employed for a year or less. Tenure has actually been increasing the past few years but this is generally because less senior people lose their jobs leaving more senior people still employed. Older workers have longer tenure on average and median tenure figures increase with age groups.
Bottom Line: Its typical for people to have around a dozen or more jobs in their life and its common for jobs to last around 4 years. People have more jobs when they are younger and later in life people tend to stick to jobs longer.