COUNTIF should work if I understand your question correctly
edit:
to expand a bit
Sheet 1 is where you want the results.
Sheet 2 Column A is where your list of A,B,C are.
Sheet 1 A1 (for letter A count) formula should be COUNTIF(Sheet2!A:A,"A")
Sheet 1 A2 (for letter B count) formula should be COUNTIF(Sheet2!A:A,"B")
etc