Hi Wes,
What version of Numbers? (Numbers menu > About Numbers)
What version of OS X/mac OS? (Apple menu > About this Mac)
Looking at the Calendar template in Numbers 3.6.2, it appears you create a new month by duplicating the Sheet containing the supplied calendar table (or by duplicating the "Blank" sheet, containing an empty calendar table), then choosing the new month from the pop-up menu in cell A1, at the top left corner.
Duplicating the Sheet where you have already entered one or more events does exactly what you should expect—it makes a copy of the Sheet, the four tables on the Sheet, and all of the formulas and contents of those four tables.
Simple solution: Start with the document as produced from the template, with nothing entered into the cells of the Monthly sheet's main table.
Duplicate that sheet as many times as you will need, then Rename the sheets to match the months (in order), and set the Month name and year in the two cells of the top left table. After setting the Month and Year, that table (and the two small tables showing month befor and month after) should be locked.
With the month and the year set, and the three subsiduary tables locked, you may want to save a copy of the document (as a Template) to use in the following year. With that done, you are ready to start making entries on the main Calendar table.
At the end of the year, close that year's calendar, open and set up a new one from the Template Chooser, and carry on.
NOTE: Don't bother making entries that repeat on the same date each month or on the same date each year before duplicating the sheets. The events will be included in the duplicate, but they will be placed in the same cell of the duplicate table, not in the cell for the same date in the next month or in the cell for the same date in the same month of the next year of the duplicated document.
Before going too far with creating a calendar document in Numbers, become familiar with the capabilities of the Calendar application included in the software bundled with your Mac. It can handle events that repeat on the same date each month or each year, and can also send you a message or an email reminding you of an upcoming event.
Regards,
Barry