a flight of stairs 一本の階段

a flight of stairs とは次の階や踊り場までの階段という意味です。階段を数える単位と考えてもいいでしょう。2:32 don't worry it's only six flights (of stairs). 降りるのはたったの六階だから、心配御無用です。

“Trumped" Starring Matthew Broderick & Nathan Lane

https://www.youtube.com/watch?v=13aqFpjneZM  1:12 I stool up a bottom of a long flight of stairs with a suitcase.
Do New Yorkers Actually Give Their Seat Up For a Pregnant Woman?

https://www.youtube.com/watch?v=9ZRkUAufZCY  7:11 you walk up the two flights of dimly lit staircases. 薄暗い二本の階段を上る
Experience Cuba in 360-degrees (English)