Counting the x's in a column

In an PDF form, there is no concept of columns (at least not unless you've created a naming convention that would make it easy to determine which fields are part of a column). 

You would need to loop over all fields in question and compare their content to what you are looking for and then increment a counter. If you are looking for fields to be filled with an X, that sounds like you are not really looking for a text field, but would be better served with a checkbox (you can make the checkbox use an "X" as the checked mark - you do this on the Options tab of the checkbox field properties and select "Cross"). 

If you are using checkboxes, the selected state is reported as "Yes", so you would use a something like this as the calculation script of the field that will report the number:

var allCheckBoxes = [ "Check Box1", "Check Box2", "Check Box3" ];
var cnt = 0;

for (var i=0; i<allCheckBoxes.length; i++) {
    var f = this.getField(allCheckBoxes[i]);
    if (f != null) {
        if (f.value == "Yes") {
            cnt++;
        }
    }
}
event.value = cnt;

If you want to keep your text fields, change the line that compares with "Yes" to a comparison with "X".  You will of course hav to change the list of fields to inspect at the top of the script.