During The Final Empire the Mistborn were described as rare, but not THAT rare. Each great house was supposed to have some. Elariel sent two just to kill Elend. The guard were "used to" seeing Mistborn in the night, so they were obviously not a very rare occurrence.
So what happened to them? In The Well of Ascension there seemed to be none except for Zane and Vin. After Zane's death Vin was considered virtually unstoppable. Were there any Mistborn with a hidden atium stash, they would have obviously tried their luck.
Did all other Mistborn die in the house war? Were there just 3 other Mistborn besides Vin and Kelsier during the Final Empire? Did all of them die because of a contaminated metal batch?
EDIT: Vin killed another one in the beginning of TWoA. Which actually makes Mistborn seem rather even less rare.