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I did the math regarding the circumferences of the different levels of fisting. The average circumference of the male hand, including the thumb, is 9.3 inches (23.6 cm). For female it is 7.7 inches (19.5 cm).

Using this, and considering things like position in the holes and compression of the hands once inside, here are the numbers for all levels of fistings:

Double fisting circumference(2 male hands)= 12.0 in.(30.4 cm)
Triple fisting circumference(3 male hands)= 14.9 in.(37.8 cm)
Quadruple fisting circumference(4 male hands)= 17.8 in.(45.2 cm)
Quintuple fisting circumference(5 male hands)= 20.5 in.(52.0 cm)

Double fisting circumference(2 female hands)= 10.0 in.(25.4 cm)
Triple fisting circumference(3 female hands)= 12.3 in.(31.2 cm)
Quadruple fisting circumference(4 female hands)= 14.5 in.(36.8 cm)
Quintuple fisting circumference(5 female hands)= 16.7 in.(42.4 cm)

Based on studying the female pelvis and the opening between the bones, on average, a woman can potentially if trained long enough to do so fit 5 relaxed females hands past the wrists inside either of her holes before it becomes too difficult to fit more (because of the bones getting in the way).