If you are refering to the actualy term "x" then you need to use COUNTIF as the function. It will ask you two questions when you use the function, the first will bethe range which you will define by highlighting the column with the x's in it. HTe next question will be the criteria which you will define as x. It will then give you the number of x's in that column.
If you want to count a series of numbers you will use the function SUM, and again highlight the column and it will sum the entire column or defined area for you.
Hope that helps. If not I may need some more info to be of further help.