The significance of numbers plays a role in many esoteric and occult traditions. Those traditions had no small influence on the development of contemporary Paganism so it isn't surprising to see it crop up within that religious demographic. Stretching further back, indigenous storytelling and folklore often evokes numbers and reputation as a sort of mnemonic for the pre-literate days when bards told tales around campfires.
In Druidry, there is probably no number more significant than three. The three drops of Awen, the three rays representing the stations of the sun, three worlds for past and present and future, three planes of land and sea and sky, gods and goddesses with three aspects, three part wisdom sayings... it goes on and on. Why three? It's been speculated by my fellows that the why isn't so important as the what it does. Where some other religious traditions emphasize either-or, this-that, black-white, refusal to conceptualize the universe in dualistic terms reveals a nuance and complexity that lends richness to Celtic and Druidic traditions in the modern day.
Before formally joining a Druid order, I would have named four as the most significant number without hesitation. The Four Elements are the foundation of my tradition, and modern Druidry incorporates them as well. As with threefold thinking, fourfold thinking avoids the problem of dualism but also acknowledges it, for the elements form pairs of contraries with one another; Air and Earth, Fire and Water. These in turn transform into one another in a rotation based on their core qualities: Air to Fire, Fire to Earth, Earth to Water, Water to Air, and so on. The metaphysics of the Sacred Four serves as a template for understanding natural cycles and the universe, as well as being a helpful memory tool.
Double the four and we get eight, a number many contemporary Pagans will immediately recognize as sacred due to its connection to the eightfold Wheel of the Year. While a modern invention, the eightfold ritual cycle of celebration is built upon indigenous Pagan traditions of Western Europe. Consisting of the equinoxes, solstices, and their cross-quarters, there is a time of celebration for every season and station of Sun. By each turn of the wheel we walk through the ancient yet ever new gateways of the seasons, repeating themes, creating and destroying, growing and decaying, planting and harvesting, time after time.
The lunar cycle also is significant and with it the number thirteen. Particularly in traditions of modern Witchcraft and Wicca, thirteen is a number of magic and mystery. Connected with Moon, it taps into the mysteries of change and transformation, of darkness and night, of reflection and introspection. Thirteen is the number of Witches in a traditional coven, which the nine foot diameter circle casting comfortably accommodates. Some consider the number thirteen inauspicious and unlucky, but the Witch, unafraid of tapping into their own power and authority, instead leverages this to spin spells and impose their will.
), I've started this thread in the games forum: 
