This may be what you need...
Open the Section Expert, select the section you want (ie Details a) click on the 'New Page After' checkbox, click on the 'x+?' formula button next to 'New Page After' and enter something like the following: "recordnumber=50"
This will print 50 records then start a new page.
Also, a possibly better solution is: you could set up a count (for example using Running totals) of your details and use that formula instead of recordnumber.