Answer: 50 Let us give an example for 50. Paint the board in chess pattern and put knights in all black squares. Only the 50 white squares are being threatened. Now let's show there will always be 50 squares threatened. Take two squares that are in the same column or in the same row such that there are two other squares between them. Our claim is one of the two of them must be threatened. That is because there is 2x2 square such that any knight in that 2x2 is threatening one of our squares. Now use these pairs to cover 6x4. Then use 6x4 s to cover 10x10 except the middle 2x2. Except that 2x2 in the middle half of the area is threatened. If we assume that there are 3 squares in the middle 2x2 that are not threatened we get a simple contradiction looking at the area that would be threatened if there was knights in those 3 squares(the are includes a 2x2 square.) So this means also half of the 2x2 in the middle is threatened. Half of the 10x10 (50 squares) is threatened.