I want to create a formula in crystal report 9 that counts only the fields that has a value equal to "A" ,is it possible to do so?

You'll need to create a calculated field in the detail section and add them together at the end.

Something like "If myField = "A" then 1 else 0"

Sum this filed up and you'll have a count of all the records where the field your interested in is "A".

or use the built in functionality called Running Totals.