Nice use of Len Roos1. The function I have placed above could also be used in a formula as it simply returns an array of 26 elements (1 for each letter) representing the count of each letter. By using the INDEX function you can return a particular element (ie letter) within the array. The index number will represent the position of the letter in the alphabet (eg B=2, J=10 etc).
So say your target cell is A1 and you want to count number of B's ...
=INDEX(COUNTLETTER(A1),2)
Count # J's
=INDEX(COUNTLETTER(A1),10)
Count # Z's
=INDEX(COUNTLETTER(A1),26)
You can also place number of each letter in cells automatically by selecting the number of cells for which you want to find letters then array entering the COUNTLETTER function.
eg
1. Select cells A1:Z1
2. Enter Formula =COUNTLETTER("Count number for each letter in this string")
3. Press CTRL+SHIFT+ENTER to confirm the formula
A1:Z1 will be populated with the count for each letter.
Example workbook attached.