According to Discovery Communications, the human face typically has 43 muscles. However, due to the complex muscular structure and the dense grouping of muscles, sometimes fewer than 43 are found on some people.
Discovery Communications states that the muscles in the face are tightly interwoven, and many of them connect primarily to skin. They are predominantly quite thin and flat, making them hard to detect, even during dissection. Also, some people develop slight variations in the muscles in their faces, some of which may be genetic in nature.
According to How Stuff Works, the popular adage that it takes more muscles to smile than to frown arises from the idea that all 43 muscles in the face are used while smiling, whereas a subset are used when frowning. However, this adage has been proven false. Some measurements even suggest that frowning uses more muscles than smiling.