It seems that there is one embodiment for each dough doll. How does it tie in with the fact that he is supposed to be one god?
When Vinayak holds two dolls in two hands, Hastar which was one also divides into two and later when Pandurang accidentally drops all the dolls out of sack Hastar multiplies with that count. What story is trying to tell is that given the availability greed multiplies and tries to consume everything. Since there is no reference of Hastar anywhere in Indian Mythology this is a creative liberty taken to suit the storyline.
Also, when all of the embodiments are destroyed does it imply the death of Hastar (I don't recall him being immortal) ?
The encirclement of dry flour keeps Hastar away from it. That has been consistent throughout the movie. It creates a spherical protective shell when person is inside of it and a boundary when out of it.
I don't think it implies that Hastar dies when all the copies of him turn to dust while trying to get out of womb. It simply means that he is sent back to the corners of the womb and can only be invoked again by presenting a dough doll.
That's why when Vinayak lures all Hastars to follow him to encircled protection, all of them turn to dust and Pandurang is able to come out of womb safely.