In the Export View dialog, under the checkbox for "Tiled KMZ for specific map type" there is a bit of text telling you how many 1000x1000 tiles it takes to cover the current view in TopoFusion. That will change as you zoom in and zoom out. Try it.
The number of map tiles shown in the main TF window doesn't matter, it only matters what number the dialog is reporting, since you will be exporting the tiles at their native resolution, while TF will shown them at different resolutions (and also kick up to higher zoom levels).
Maybe that makes sense?