Create multiple file from single txt file

I have a single input file that needs to be replicated multiple times with a little change in every file i.e every 30th line in every file is to change. For e.g. My main text file name is D0.txt which I need to replicate multiple numbers time. and in that line 30 is "variable D1 equal 0.0 " Now for the first time when I replicate this file I would like to change that string to " variable D1 equal 1 " and then save the file as D1.txt. Similarly, I would like to create suppose 5 files then I would like to loop it 100 times. so files are saved in a folder as D0.txt D1.txt D2.txt and so on with each of their line 30 as "Diameter = $n" sample text file format is attached below. and desired format of files in folder.