Question of a C# programm with ascii code

C# doesn't use ASCII by default - it uses Unicode which is wider (16 bit instead of 7 or 8 for Extended ASCII) . when you use Console.readLine the string you get is a Unicode string, containing Unicode chars. For "true ASCII" you would have to use an array of bytes, and translate the Unicode:

string s = Console.ReadLine();
byte[] bytes = System.Text.Encoding.ASCII.GetBytes(s);

But ... for most input that's irrelevant, and if you're a beginner this is probably your homework to count characters.
Easiest way? Sort them. Then the repetitions are all next to each other and you can count them with a simple loop:
1) Set count to zero.
2) Set currentChar to '\0'
3) Loop through each character in the sorted data
3.1) Is currentChar is the same as the loop character?
3.1.1) Yes: increment count
3.1.2) No: Is currentChar '\0'?
3.1.2.1) Yes: increment count, set currentChar to loop character
3.1.2.2) No: Print currentChar and count, set count to 1, set currentChar to loop char
4) After loop, print currentChar and count