Hi,
I think it's smth like  LEN(Field)-LEN(SUBSTITUTE(Field,'A','')) & 'A'
same for B, C etc
but instead of Field should be ARRAYJOIN(Field)  or CONCATENATE(Field)
  or (values) instead of (Field)
and if your A, B  are words longer than 1 char, you should divide on it's LEN
if you have many letters, it's wise to put them all in a vertical list and use a formula to create a formula.


Note: hardcoding values in formula is a bad idea sometimes. If you create new linked field to a new table and copy  this column, and add Count field in new table, you get pivot table close to your desired result

Thx Alexey!

Sorry for the late reply, did what you suggested and helped!

Thank You!