There is no "records per page" setting. It will print as much as will fit.
Most reports would show about 40 records per page.
The only way I would know to do it would be to make your detail section
exactly half of the height available after you factor in margins, page
headers, report headers, footers, etc.
If you get rid of all the headers, just subtract your margins from your page
height and divide by 2. Then make your detail section that height.