You can use numbers to inscribe on candles, to collect meaningful numbers of objects, to arrange things in shapes of numbers, to make symbols of any sort, and just to notice in the world! Here are some number meanings, these are taken from Scott Cunningham's Wicca: A Guide for the Solitary Practitioner (which is a pretty good first book, I must say!), with the starred ones being ones I added.
1--The universe; The One; the source of all, **oneself**.
2--The Goddess and the God; The perfect duality; projective and receptive energy; the couple; personal union with deity; interpenetration of the physical and spiritual; balance.
3--The Triple Goddess; the Lunar phases; the physical, mental and spiritual aspects of our species.
4--The elements; the Spirits of the Stones; the winds; the seasons.
5--The senses; the pentagram; the elements plus Akasha; a Goddess number.
7--The planets which the ancients knew; the time of the Lunar phase; power; protection and magic
8--The number of the Sabbats; a number of the God, **the infinite number**.
9--A number of the Goddess, **a fertility number**, **a motherhood number**.
13--The number of Esbats; a fortunate number.
15--A number of good fortune.
21--The number of Sabbats and Moons in the year; a number of the Goddess.
28--A number of the Moon; a number of the Goddess.
101--The number of fertility.