For example, a button named "Purchases". When ever the "Purchase" button is clicked a new number field is automatically created ...
look at your problem in this way:
When ever the "Purchase" button is clicked a new record is automatically created
That makes sense. But what I am trying to do is keep track of purchases for each month so my "record" would be the month of January and within the month of January there would be several purchases. I wanted to make a button that would create as many entries as I needed for each month instead of having to predefine a certain amount of entries.
To keep trace of montly purchases, all you need is a date field.
You can't create new fields, but only new records.
You could have a table for the items. Then have a table for the purchases. Put a portal on the item record to enter purchases. The purchase table could have any
number of fields of information you wish to track for the purchases. But you would have multiple purchases for each item recorded.