There are 12 classic signs of the greek zodiac, though some like to throw in an extra one making 13 in total.
The number of signs equal, not coincidentally, the numbers of months in the year, making it possible to assign one to each person's birth date. However, the zodiac itself is more complicated than that and instead works in degrees. You can, for example, be more Taurus than Gemini, depending on the proximity to one star-sign's date range over another.
Some astrologers claim there is a 13th sign of the zodiac called Testudine which is the sign of the tortoise. This sign is said to be confident, strong and strong-willed. Testudine's best match is a late-month Geminiian.