It depends. I'm a government contractor, where the contract goes up for renewal every 5 years. If a new company wins, they usually take on all the existing employees, and about the only thing that changes is who the paycheck comes from, and health insurance/401k and other benefits. But we get to keep our seniority.
As of October, I'll have 31 years of seniority. But, technically five employers. Although the first one was actually one of those Chrysler-esque "mergers of equals", when Boeing took over McDonnell-Douglas.
So, I didn't answer, because I didn't know if I should put one, or five (or four if I count McDonell-Douglas and Boeing as one).