Since you are so new to FileMaker, I strongly suggest you invest some time in reading up on the subject or participating in some on line tutorials. There are a number of such options available.
I sounds like your stock fields are different copies of exactly the same number field. That would explain the behavior you are seeing. Enter Layout Mode and look at the field names. You may have to turn off the Sample data option selected in View | Show | Sample Data before you see field names instead of sample data. I suspect you'll see the same field name for each field except XL in Stock.
You can double click a field to select a different field if you have already defined fields for each stock level. If you have not, then you'll need to define these fields first.