Writing content in a text file dynamically based on an input variable

I have a text file in which you have the following statements in the middle of text file(there are other statements starting with set word).

set TC_SYS_01       0
set TC_SYS_02       0
...
set TC_SYS_38       0

Now based on an input from GUI(say 500),i have to copy all of contents of above text file only with the following lines changed to

    set TC_SYS_01       0
    set TC_SYS_02       0
    ...
    set TC_SYS_500      0

----------------------------------------------- My Code:------------------------------------------------

content = fileread('run.txt');
length = regexp(content, '(?<=set TC_SYS_\d*\s+)[01]');
% the above line is just to know how many lines of set TC_..... are present;
% input is 500,
%%%%%%%%%%%%%%%%
fid = fopen('run.txt','r');
tline = fgets(fid);
if strcmp(tline,'set TC_SYS_%d',length)
index = ftell(tline);
end
for i = length:N
% fprintf(index + 1) ??
% how to write statements after this ?
end
%%%%%%%%%%%%%%%%%
fid = fopen('run1.txt','w');
fwrite(fid,??);
fclose(fid);

If possible from the original text file which has those 38 lines of "set ...",i want to copy rest of the stuff into a new txt file & iin the new txt file i write afresh N lines of "set ...."

[Geoff Hayes]

Luffy - to be clear, you want to insert lines of text into the middle of your text file. Is this correct?

[Luffy]

Yes. If input is 500 then insert 500 lines, If input is 40 then insert 40 line,