In Isaiah 6:2, the word "twain" refers to
two.
The verse reads: "Above him stood the seraphim. Each had six wings: with two he covered his face, and with two he covered his feet, and with two he flew."
So, each seraphim had six wings, using two to cover their face, two to cover their feet, and two to fly. "Twain" is an archaic word for "two."