How do I count a specific letter at the begining in an input box that is repeated 10 times?

The Text property of a TextBox returns a string containing the input so far - and a string can be treated as an array of characters:

string greeting = "Hello World";
Console.WriteLine($"{greeting[0]} {greeting[6]}");

Will print the first letter of each word:

H W

So to find out how many times a character is repeated at the beginning of a string is simple:

1) Create a variable to hold the "compare character" - call it firstChar
2) Set firstChar to the first character in the string
3) Create a variable to hold the identical values count called count, set it to zero.
4) Use a foreach loop to look at each character in the string.
4.1) If the character is not the same as firstChar, exit the loop using break
4.2) Otherwise, increment count
5) After the loop, count will contain the number of identical characters at the beginning of the string.