How to add numbers to string in a file?

Question

I have a text file called sample.txt and its contents look like

 

John Howard Carpenter @#@ born in Carthage, New York, 

to mother Milton Jean (Carter) and father Howard Ralph Carpenter.

His family moved to Bowling @#@, Kentucky, @#@ his father, a professor,

was @#@ of the music @#@ at Western Kentucky University.

 

How do I replace all @#@ by the numbers starting from 1, then incremented by +1 till no @#@ is left in the file.

 

I've tried

string x=File.ReadAllText(@"C:sample.txt");
string y="@#@";
int i=1;
MatchCollection matches = Regex.Matches(x, y, RegexOptions.IgnoreCase);
if (matches.Count>0)
            {
                foreach (Match m in matches) {
                    string n=x.Replace(y,i.ToString());
                    i++;
                    File.WriteAllText(y,n);
                }
            }
MessageBox.Show("Done");

It does not do anything

BTW: The expected output is

 

John Howard Carpenter 1 born in Carthage, New York, to mother Milton Jean (Carter) and father Howard Ralph Carpenter. His family moved to Bowling 2, Kentucky, 3 his father, a professor, was 4 of the music 5 at Western Kentucky University.