Is separate gst required for different state business?

Supply definition as per section 7 says that transfer is also a supply and hence whenever you are transferring to your branch you have to pay tax

further, once you pay tax and if you don't take any credit then it will be a loss for you

Hence apply for registration in each state separately in order to avoid the loss of input credit

I am asking this question because,we got a IGST bill from  Karnataka state. But they are having a brach in Kerala. Can we accept this bill?

you can if GST number is correct