Do we have to give practicals again if I am writing an improvement for class 12??
No dear you don't need to give practical exams again. According to guidelines laid by cbse, Students simply need to prepare for theory exam, as their practical marks will be added in the improvement exam results. If the candidate has failed in practical, then only he/she need to appear for both theory and practical exam, irrespective of the fact that he/she has already cleared the theory examination.
Also you don't need to give any internal exam for 20 marks in case of English and Maths .You don't need to worry either about 20 marks of internal examination because your marks will be calculated out of 100 even if you are giving exam which is just of 80 marks. Marks obtained by you out of 80 will be converted to marks out of 100 .
If you score 72 marks out of 80 , then in the result your marks out of 100 will be (72÷80)×100 = 90 marks.
Cbse will take of these things as you are an improvement student. You don't need to worry at all.