Is it not supposed to be "majaqis" since it's plural?
No, skorverdon is considered a singular noun, with a declension according to 3ter. "Valoti" is simply a modifier. Think of it as "How many of the men".
I think so too - according to the vocabulary, skorverdon results in "genitive, usually plural" ... as with Skorverdon valoti ēdrusi? — How many men are sleeping?