Throughout the book, the number seven has been used in many instances. The main ones that I remember are Okonkwo’s seven-year exile from his village, the locusts reappearing every year for seven years and then disappearing for another lifetime, and the seven market weeks until the gods would no longer allow a man to defy their laws. But why the number seven for all of these? According to Nahmanides (a leading medieval Jewish scholar), seven is the number of the natural world. There are 7 days in the week, 7 notes on the musical scale, and 7 directions (left, right, up, down, forward, back, center). Other things associated with seven are seven days in Creation, seven deadly sins, seven seas (the Arctic and Antarctic, North and South Pacific, North and South Atlantic, and the Indian Ocean), and seven visible moving celestial objects (Mercury, Venus, Mars, Saturn, Jupiter, Moon, and Sun). So, there are an immense amount of associations with the number seven outside of the novel (and I’m sure there are many more that I failed to mention).
Although I brought up many random things tied to the number seven, many are associated with religion (many more biblical ones that I didn’t reference), in particular Christianity. The use of the number seven in the novel just shows the similarities between the Igbo religion and Christianity, although both the natives and the missionaries don’t really see resemblances between their beliefs.