Is the creature in "The Thing" a single creature?

In that case does that mean every person imitated by the thing was a self-reproduced creature of the same species.

Yes...this precisely.

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.

From the script

                                MACREADY
                     Watching Norris in there... gave me 
                     the idea that maybe every part of 
                     you bastards is a whole. Every piece 
                     of you is self-sufficient, an animal 
                     unto itself. When a man bleeds it's 
                     just tissue. But blood from one of 
                     you Things won't obey. It's a newly 
                     formed individual with a built-in 
                     desire to protect its own life. When 
                     attacked, your blood will try and 
                     survive -- and crawl away from a hot 
                     needle say.