Stairs vs Stairmaster vs incline treadmill

I am in the midst of the 8 week mountaineering training plan and of course it’s calling for many “uphill” exercises. The highest stair cases that I have access to is about 10-11 floors high, there is an accessible outdoor staircase as well. Many of my training exercises however I do on a treadmill or stairmaster or both (to tackle the boredom factor…).

My question now is, what is the minimum of flights or stairs where going up and of course down that stair case (although I could take an elevator down but maybe the step down is also important?) has potentially a better benefit than being on a stairmaster/treadmill where the step moves away underneath you?

Thanks in advance,
Andy