Seven
Seven is an expression of time, development, and completion; according to the Jungian analyst Kenneth James, the number represents "the active fulfillment" (Speaking of Jung, ep. 56).
The "original" lunar time was measured by the number seven (in contrast to the later solar time measured by the number twelve). Lunar time is, in a sense, the "natural" time; anyone can measure time by looking at the moon.
According to von Franz, seven is the number of development, due to the seven planets, which in astrology represent human totality. The week has seven days, and then it begins again (associating the number with a completed time cycle); the seven planets come and go, and so forth, linking the number seven to ongoing development. Seven as a number can represent "the highest rung," or the goal.
The number three signifies a dynamic that comes to rest and completion in four; these numbers in combination express both aspects. – The relationship between, or "the problem of," three and four is extensively described by Jung in Psychology and Alchemy; the dynamic of seven and eight is a "differentiation" of this, according to von Franz (The Feminine in Fairy Tales, p. 133). In brief, the issue pertains to the difficult step from one to the other; "the final" that brings completion refers to the shadow.
The number is thus a recurring expression of human development. Anthony Stevens exemplifies this with "the seven terraces of the ziggurat, the seven rungs on the ladder in Mesopotamian tombs, the seven metals in Mithraic rituals, the seven steps to the alchemical bath, etc." (Privata myter, p. 204).