According to wikipedia, jaffa cakes have an average diameter of 54mm. The top jam layer has an average thickness of 4mm, and the covering chocolate layer has an average thickness of 1mm, totaling in 5mm of thickness. For simplicity purpose, let us assume that a jaffa cake is a perfect cylinder, which will end up with more volume than a realistic jaffa cake giving us a certain security coefficient to work with. With before mentioned information, we end up with all the data required to give us a result of a single perfectly cyllindrical jaffa cake volume - 11445 mm^3, which we will approximate to 11,5 cm^3. According to wikipedia, male human mouth on average can hold 71,2 cm^3, while a female mouth holds 55,4 cm^3. Dividing volumes of mouth to volumes of the jaffa cakes, we end up with the following - an average male human mouth can hold 6,2 jaffa cakes, while an average female human mouth can hold 4,8 jaffa cakes. Because we assumed perfect cylindric geometric shape of jaffa cake, and the non-elasticity of jaffa cakes, we can approximate the results to the higher end. Therefore, it is safe to say an average human male can hold at least 6 jaffa cakes, while an average female should be able to hold 5 or more jaffa cakes in their mouths.