having several files with a given field what is the formula to get the sum of the fields ?

Try to use the visual formula editor. There you can click on the linked Table, then choose the field an pick "sum".

I have a record and some fields that are populated and others are blank blank.  I want to count the fields that are populated.  I have created a count field to facilitate this.  What is the formula to count the re

cnt(select table[field )= null])