What shade of brown do I need to buy to get this hair colour?

To avoid getting the wrong color or having it turn out to be the wrong shade... I would go to a salon. Your hair may not create the exact same shade you see on the box. Everyone's hair takes differently to the dye. Or if you do not wish to get it done at a salon, at least seek professional advice. An expert will be able to guide you toward the perfect color and brand. I hope this helps!