In recent years it has become more common for workers to have a few careers or to work at several different companies. In the current economy this will probably become more common. Fifty years ago your grandfather probably worked for the same company his whole career. But with the changing demands of the economy this is very unlikely.
As new technology is developed and the demands of the jobs change workers are forced to job hop. Indeed for advancement in your field or part of your profession that you work on projects that it may be necessary to job hop. So if changing jobs more frequently has become the norm is it possible that job-hopping can be detrimental to you career? I believe that it can.
Employers expend a lot of time, energy, and money recruiting staff so they may question someone who has changed jobs annually for last ten years. As a human resources officer reviewing a resume I would be looking at the changes in the jobs to determine the patterns. So if you moved to a different firm after you obtained your degree or the move was a promotion or advancement in your career then I would probably not be as concerned. I also believe that people early in their careers tend to change jobs more frequently until they have settled into a job that provides them with the opportunities for additional training, experience, and advancement.
So how many job changes are okay? Well that depends on your circumstances. But I believe that employers are not as willing to provide the additional training required to advance in your field if they are not sure you will stay long enough for their company to benefit. So consider your job changes carefully.