Hello Rupesh,
you didn't write what kind of action you are doing I guess you are shaping.
you need to be able to send 12000 bits in a Tc if credit has been accumulated.
Each Tc you get 8000 bits that can be held in two buffers Bc and Be.
if Bc is 8000 to be able to handle 12000 bits in a single time interval the second buffer size has to be:
Be = 12000 - Bc = 4000
Hope to help
Giuseppe