Then do not use n as counter, but another one:
iFile = 1;
...
base_name = sprintf('%s_%i.jpg', org_name{end}, iFile);
full_name = fullfile(newfol, base_name);
imwrite( PR_NB, full_name) ;
iFile = iFile + 1;
By the way, you can omit the "char()", because fullname is a char already.