No rule for this one
No ending or word group predicts the article of Stuhl. Learn it as a pair and say it out loud: der Stuhl.
der Stuhl
It's der Stuhl. Stuhl is masculine, plural die Stühle.
Wir brauchen vier Stühle.
We need four chairs.
No ending or word group predicts the article of Stuhl. Learn it as a pair and say it out loud: der Stuhl.