Preview mode displays your data one page at a time. Do the 16 records fill the page?
To see subsequent pages while in preview mode, click the book control in the upper left corner of the task bar. That control flips you through records in browse mode, but flips your through pages in Preview mode.
No, there are eight records on the first page and eight records on the second page when I'm in preview mode.
And two pages are all you get in preview? That's how I understand your post but want to be sure. This is one of those issues that can be difficult to figure out because I can't see what you are seeing.
You should see the same records that are present in your found set, that's how filemaker is designed to work. If you don't see all the records, then something unusual is going on and that can be hard to diagnose through a forum discussion such as this.
What do you get when you actually print the file or make a PDF?
Preview is supposed to predict what you will get when you print the file, so that experiment may tell us something.
Are the 16 records you can see in preview the first 16, the last 16, or ?
I'm not sure if you've answered this question or not: "Do 8 records fill the page?"
You could also create a new layout and see if you get the same behaviour. (Sometimes a layout gets corrupted.)