Sixteen is two raised to the fourth power, a doubly even number that Indian tradition made into a figure of completeness. The moon is said to have sixteen kalas, sixteen digits or phases, and a being possessing all sixteen is complete; Krishna is called shodasha-kala-purna, full in all sixteen parts. The classical life is marked by sixteen samskaras, the sacraments from conception to cremation, and the sixteen-part puja, the shodashopachara, offers a deity sixteen services in turn. Early Buddhist and Jain geography counts sixteen Mahajanapadas, the great states of the Ganges plain in the Buddha's time, a list with real historical content. The recurrence of sixteen as the measure of a whole, rather than the ten or twelve of other cultures, is a distinctively Indian habit, rooted in the phases of the moon.
Modern numerology treats sixteen, as this article already notes, as a karmic-debt number, but the specific reading, ruin of the ego followed by painful renewal, is one Cheiro's 1926 Book of Numbers ties directly to the tarot card it shares a number with, the Tower, struck by lightning; Balliett's 1908 chart carried no such compound reading, and it is Cheiro's book specifically, rather than Balliett's or the later Campbell and Jordan systematisations, that is generally credited with linking particular compound numbers to major-arcana imagery of this kind.