Tax ID number: one per employer, or one for life?

Is a TIN tied to a specific employer, so if a foreigner changes jobs, a new one would be issued? Or does a TIN issued to an individual become the one and only TIN for that individual, for life?

I was working with a lawyer and the topic of my old TIN (from when I worked and paid taxes in Thailand, some years ago) came up. He said my old TIN would not be used with a new future employer.

This advice doesn’t make sense to me. For one thing, it would seem to imply filing multiple tax when a foreigner switches jobs during a given year, because I only see one space on the form for a TIN — not to mention trying to determine correct tax rates when income has been split across separate returns. The issue of non-employment income (interest investments etc.) complicates that even further.

That seems totally unworkable. On the other hand, this certainly wouldn’t be the first thing I ever encountered in Thailand that doesn’t make sense to me.