I need to place an order for fresh dark red roses to place between the tiers of my brothers wedding cake for next weekend and was hoping someone can help me. We will be using fillers and babies breath. The cakes are 12 and 9 inches. My mother said one dozen would be enough she is making the arrangements. TIA.

If your mother is making the arrangements and thinks one dozen is enough, then that should do it. If you feel the need to get two dozen and then you will have an extra dozen to decorate the table with. Of course that also depends on the price of them

see this similiar topic where i explain how to "do the math".

http://cakecentral.com/cake-decorating-ftopict-680086-math.html

without knowing exactly the height you're trying to cover and the size of the roses it's hard to give you an estimate. Hopefully this will help.