Expressions like “twenty-odd years,” “a dozen-odd people,” and “two hundred-odd mistakes” are usually written with a hyphen before the “odd” to indicate that the exact number is unknown—perhaps a bit higher than the stated number. If you omit the hyphen, as in “a dozen odd people attended my birthday party,” you risk giving the impression that the people who came were odd rather than that you can’t be sure of the precise number of your guests.