How many months formed a year in the calendar of Romulus (the original Roman calendar)?

The Romans themselves usually described their first organized year as one with ten fixed months, a decimal division fitting general Roman practice. There were four months of "31" days—March, May, Quintilis, and October— called "full months" (pleni menses) and six months of "30" days—April, June, Sextilis, September, November, and December—called "hollow months" (cavi menses). These "304" days made up exactly 38 nundinal cycles. The months were kept in alignment with the moon, however, by counting the new moon as the last day of the first month and simultaneously the first day of the next month. The system is usually said to have left the remaining 2 to 3 months of the year as an unorganized "winter", since they were irrelevant to the farming cycle. Macrobius claims the 10 month calendar was fixed and allowed to shift until the summer months were completely misplaced, at which time additional days belonging to no month were simply inserted into the calendar until it seemed things were restored to their proper place. Licinius Macer's lost history apparently similarly stated that even earliest Roman calendar employed intercalation.