How many planes of symmetry does a cuboid have?

It depends on which type of cuboid we are talking about. If it is a CUBE (a special type of cuboid), then it has nine planes of symmetry.

If it is a cuboid with length, width and height all different, then it has three planes of symmetry.

If it is a cuboid with two equal measurements (say width and length), then it has five planes of symmetry.