Yes, Sajid is right. It depends from many, many factors, but on an average eache unit will higher up Hb for 1 g /dl. This means that if your Hb is 6,9 g/dl it will be around 9 g/dl.
It is recomandet to not underline 9 g/dl befor transfusion. Otherway your enteral iron absorbtion will rise up a lot.
God bless you