Enter layout mode.
On the Inspector's position tab, click the units util you get points instead of inches or centimeters.
Click the header layout part.
Edit the height shown for the header to reduce its height by 1 point.
Preview the report to see if you now get 10 rows of labels instead of 9.
Repeat if necessary until you get 10 rows.
You can also try leaving the header unchanged and reducing the height of the body by 1 point. (Which results in a reduction of 10 points for the total height of 10 rows of labels on one page.)
What's at issue:
Most printers cannot print all the way to the bottom edge of the page. The distance between the bottom edge the the lowest point that your printer driver permits printing is slightly larger than the distance between the bottom edge of the last row of lables and the edge of the labels page. This is sufficient for FileMaker to determine that the last row of labels cannot be fully printed and it then generates a page break to start the 10th row on the next page. Usually, very small "tweaks" to the dimensions of the layout parts can adjust the layout enough to avoid the undesired page break will still placing the labels accurately on each label.