Can a Knight check 9 pieces in chess?

Answer 1

No. The highest number of pieces a knight can threaten (not "check") is 8. A knight can move up to 8 squares at any given point (sometimes fewer if the knight is near the edge of the board or obstructed by pieces of its own color).

The highest number of pieces a knight can threaten just after moving is 7, because one of the eight pieces it can attack is the square it just left, which thus cannot be occupied by an enemy piece. However, the opposing player could then move a piece into that square and so the knight could threaten 8 pieces at once.