Why does the cover maker need my final page count?

The spine width is calculated from your page count and paper type — more pages (or thicker paper) means a wider spine. Since the cover is one continuous wrap (back + spine + front), the spine width sets where the front and back panels sit. Get the page count wrong and the whole wrap is the wrong width: your spine text drifts onto the front, or the printer rejects the dimensions.

So use your final interior page count. If your interior isn't finished yet, use your best estimate to design, then update the page count before you export — and re-export the cover if it changed. The Interior Formatter reports your exact page count and writes it back to the cover for you, which is the reliable way to get this right. See Can I change the page count after exporting?