org-footnote-auto-label

org-footnote-auto-label is a variable defined in `org-footnote.el'.
Its value is

t

Documentation:
Non-nil means define automatically new labels for footnotes.
Possible values are:

nil Prompt the user for each label.
t Create unique labels of the form [fn:1], [fn:2], etc.
confirm Like t, but let the user edit the created value.
The label can be removed from the minibuffer to create
an anonymous footnote.
random Automatically generate a unique, random label.
plain Automatically create plain number labels like [1].

You can customize this variable.