allout-numbered-bullet is a variable defined in `allout.el'.
Its value is

  • Automatically becomes buffer-local when set.
  • This variable is safe as a file local variable if its value
    satisfies the predicate `string-or-null-p'.

String designating bullet of topics that have auto-numbering; nil for none.

Topics having this bullet have automatic maintenance of a sibling
sequence-number tacked on, just after the bullet. Conventionally set
to "#", you can set it to a bullet of your choice. A nil value
disables numbering maintenance.

You can customize this variable.