Numerology

Numbers have many meanings magically. The but here are some of the more universal ones. When combining numbers the usually go multiplicitively but meaning can also sometimes be found in the individual digits or in the sum or product of digits. These numbers can often be found in ritual magic or the glyph representation of spells.
1. Unity. often used to combine elements. circle to bind other elements together
2. Duality or Choice. Often represented by a V in magical symbols or circles or by a single line bisecting a symbol. Two can represent duality and choice but also augment something else into meaning its opposite. 
3. Fate. Fate comes in threes, three heroes, three villains, three arcs of a journey, it just happens. Due to the connection between fate and magic it is also a magically powerful number, but those who seek true ower opt for 6.
4. Death. Often seen in rituals or spells to create or control minor undead, it is also representative of the end not just of life but of any or all things, thus the four horsemen of the apocolypse. Things pertaining to undead transformation such as lichdom or vampirism where you maintain a sense of self are more associated with 32 or 44. When power word kill is cast by a wizard its incantation has four syllables.
5. Unholy/Lower Planes. Pentagrams, need I say more.
6. Magical Power. Some say that fate and magic are two sides of the same coin, others say that the number six reprents arcane magic because it is between Divine and Unholy, others say that its because you can fit 6 equalateral triangles inside a hexagon. If one is doing an arcane ritual that does not use the magic of either the upper or lower planes there is almost always a hexagon present. Many elven cities are shaped like hexagon for this reason and the same is true for Brickerton, though depending on who you ask they may have just been copying the elves.
7. Divine/Upper Planes. Seven is the divine number, it is said that the world was created in 7 days and that there are seven layers of heaven. A seven pointed star is usually used in rituals pertaining to the upper planes. Dwarven military units are organized into groups of seven to honor Gott and their temples all have seven sides. While the number seven can be used for all the upper planes it is especially associated with God the creator of the world. Other upper plane dieties may sometimes use 14 because it intersects the divine and choices.
8. Life or Regeneation. Turned on its side it also makes the infinite, scholars debate wether it is the infinite or the combination of duality and death that create the meaning of the number. Octogons or eight pointed stars are often used in healing rites, many elven templs have an eight sided room with a seven pointed star carved in the floor to aid with divine healing.
9. Prophesy or Greater Fate. All true prophesies have 9 lines, usually 3 stanzas of 3 lines, aab bbc cca, though not always depending on what the prophesy is about.
10. Order. Order is actually rarely used in magic since most magic being used wants to change something about the world around it. But it can also represent Law, which is used in certain magical oaths that don't gain power from other sources, such as fey bargains and devil contracts.
11. Chaos or Transutation. Chaos, change, or transmutation is part of many magics but is used sparringly, unless one is attempting to cause choas or randomness such as wild magic candies or a wand of wonder.
12. Combination, fate x death, duality x power. 12 can be used for fate, death, power, choice or their opposites. This makes twelve the most universal magic number. 
13. Blood Magic. The most powerful blood magic is always done on the 13th day of the month with the most dangerous being December 13th, the 13th day of the 13th month.
17. Far Realm. The strange 17 sided shapes used in Far Realm magics are rarely symetrical in nature and are changed based on what far realm entity you are trying to draw on, this makes it very easy to mess up and is one of the reasons why Far Realm magic is banned by the Order of Mizzet.
19. Fey. While 19 sided shapes are just as easy to mess up as 17 side ones there is actual use for a regular eneadecagon in the magic rituals used to make fey bargains. Irregular 19 sided shapes are instead used for different Archfey.
21. Fate x Divine. When the dwarves send expiditions into the underdark or guard details for important peoples they often use 21 warriors in an attemt to have Gott watch over their fates.
32. Undeath or Reincarnation. death x rebirth, fate + duality. Often represented by a square insdide an octagon rather than a 32 sided shape, it is used in the Lichdom ritual as well as the creation of the first vampires and the creation of other undead that maintain their soul rather than it passing on.
35. Elemental Planes, unholy x divine. At the intersection of the positive and negative planes you have the elemental chaos. 35 is often represented by a 7 pointed star overlapping a pentagon. Other parts of the ritual or marking in the spaces between lines typically determine which elemental plane you draw from or if you draw from all 4.
38. Shadowfell, duality x fey, fate + rebirth. The shadowfell is often invoked in spells or rituals that cause darkness, fear, or misery.
50. Hells or Devils, Unholy x Order. Used in the creation of devil contracts or rituals that call upon the magic of the nine hells.
55. Abyss or Demons, Unholy x Chaos. Used to call upon the magic of the Abyss, if not to summon demons then sometimes in madness or corruption rituals.

Type

Metaphysical, Arcane