1. An image processing apparatus comprising: an image information acquisition unit that acquires image information representing an image obtained by reading a recording medium containing one or a plurality of detectable substances; an extraction unit that extracts an image corresponding to the detectable substances from the image information acquired by the image information acquisition unit; a computation unit that calculates a number of detectable substances in the recording medium overlapping each other when viewed from a direction perpendicular to a surface of the recording medium based on the image corresponding to the detectable substances extracted by the extraction unit; and a memory that stores the number of detectable substances calculated by the computation unit.