I suspect AC is counting treads and not risers, as we tend to do in the UK
Hm.. it appears Jonas is also in the UK.
The right representation would be as per attached, where we have 17 as the last step and 18 as the landing, not a step, only the landing, but is still numbered for there is a riser there.
Well, as you figured out by now, the most obvious solution is to make a stair one riser short, add "18" as simple text to the last landing and group text and stairs together. This is by no means a dynamic solution, as "18" won't change or move if the stair is edited.
Other rout might be to edit GDL object of a specific stair to allow it to do what you want (show/hide last step, but not the number, or alternatively, just add the extra number for the last riser). This will work fine with a specific stair object, but will not affect other stair objects you use and will not affect Starimaker.
If you use the same stair object a lot, I can help you with a specific object/stair (even with all the typical ones in the library, as there are only 6 of them), but to make it scalable (i.e. be able to do that on all stairs we can make through Stairmaker) — we would need to modify the Stairmaker add-on, and, honestly, it would be easier to create new stair tool from scratch than to change the old Stairmaker.
PM me if you need help with this.