I have a planning and scheduling sheet with columns:

Name - start - end - event - facility

For facility, via the combo list, users can choose for variables:

(C) Car
(B) Beamer
(M) Meeting Room

I want to add another column that mention the value 1,2, etc representing the order in time that a value Facility has been chosen:

John C - 2 (chosen after Marc)
Keith B - 1 (chooses as first person facility B)
Marc C - 1

If the facility is erased or another facility is chosen, then the value changes accordingly.
Marc changes facility to B:

John C - 1 (become as first chosen)
Keith B - 1
Marc B - 2

This is on a per day basis. The date is mention on one of the cells on top of the day.

Bart