As we can seen from the reaction:
3a+3B-->3C
3 moles of B react with 3 moles of a to give= 3 moles of C
3 moles of A reacts with =3 moles of B
2 moles of A reacts with =2 moles of B
So to react with 2 moles of A, 2 moles of B are needed.
So the answer is 2 moles of B will you need to convert 2 moles of A into as many moles of C as possible.