+---+
        |   |
    +---+---+---+
    |   |   |   |
    +---+---+---+
        |   |
        +---+

As depicted above, a X-pentomino is formed by using a 3x3 grid with the four of its corners removed.

Determine the maximum number of such X-pentominoes that could fit inside an 8x8 square grid without overlapping.