So there is no difference between the following two?我想换个工作 is literally "I want to change a job", not "jobs".
In our conception, we are not satisfied with a certain job, and want to switch to another one. We are not switching many jobs at one time.
个, the most common measure word. When the number is "one", and is not stressed, 一 is often omitted. "一个 something" becomes "个 something".
Saying 我想换工作 is also idiomatic, because we usually don't need to discuss the number at all.
Absolutely, the "jobs" means the job one changes from and the job one changes to. So it's two jobs, plural.Just to clarify a point.
"change jobs" does not mean changing jobs multiple times. It involves just one job switch.
Ex: He changed jobs recently. - 他最近换了工作.
Ex: He changed jobs several times (or: He changed several jobs) in the last year. - 他在过去的一年里换了几份工作.
Ex: He changes jobs frequently. - 他经常换工作.