The idea of one dozen roses of flowers of any other variety has to do with the special character of 12 as a number. A dozen or twelve is an important number in both ancient and modern history and practices. There are twelve signs of the zodiac, twelve months in a year and a number of other important things which tend to be organized in twelves or a dozen (a crate of eggs included). It is safe to assume that one dozen roses is not associated neither with eggs nor the Zodiac, the association with the twelve months of a year seems to be more relevant where the significance of one dozen roses (and not only roses - other flowers will do as well) is indicative of one flower for every month of the year.

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Despite the tradition, there are people who like to do things their own way, You could for example give roses in a number representing the time in months or years that you have spent with your partner, the number of birthdays the person has celebrated or any other significant number that is important for the person who will receive the flowers (and the person giving them).

But - a safe bet will always be one dozen roses. Red. 

 More on one dozen roses here. And another site talking about one dozen roses.