A flight of stairs contains 12 steps in that single flight of stairs.
If you have 20 flights of stairs then you would be climbing up 240 steps.
Climbing up flights of stairs is a good workout and burns off more calories in the same amount of time than walking.
Walking is good for exercise as well but if you can climb stairs instead of taking an elevator then you'll be improving your health.
I try to take the stairs in buildings when possible instead of taking the elevator because I keep my weight down and have improved health.