This is probably not FormRangers fault, but a limitation of Forms.
I have a simple form with only a single question and only two alternative items (in the Sheets column Available Sessions I have two rows) to choose from. There are 12 available seats, and when it reached 12 seats for one of the items, it was properly removed from the list of available items in the Form.
But when the second item also reached 12, the Sheet column for Available Sessions was emptied, as expected, but in the Form the question still has the previous value for the item, despite me running FormRanger manually several times.
This of course, left the Form open, and there was a 13th person able to book a session, as it was still listed in the Form.
So FormRanger is most likely not able to remove the last item in a multiple choice list.
Instead I had to add a "Session 1 is fully booked" and "Session 2 is fully booked" to the formulas in the Sheet, so FormRanger has a real value it can push to the Form.
It would have been cool, of course, if FormRanger had a variable I could set, which would only show when there are no items pulled from the Sheet! It'd have to be an open text box, so FormRanger admins could enter any type of information there.
Something like "There are no bookable sessions".
This would then show as the only available option.