In that case does that mean every person imitated by the thing was a self-reproduced creature of the same species.
As you state, there are multiple instances of a creature being destroyed.
Each instance of the "Thing" is independent of the others but that's not to say they don't function together or in concert to achieve their goal.
There is an extensive answer on our sister site, Science Fiction and Fantasy, addressing this precise question.
Rather than copy and paste it as a whole (and it's very long) I will quote the basic answer and link the remainder below.
This is the million dollar question, as far as The Thing is concerned. Unfortunately, not only has the answer ...