You'll probably have to play with this a little if you have group footers, but if there are no group footers, here's what needs to be done.
1. Create a Running Total to count some unique field for each detail record. I'm going to call this formula {#RecordCount}.
2. Open the Section Expert for your details section, and click on the formula button for "New Page After". Enter the following formula:
{#RecordCount} mod 25 = 0
This will create a page break after every 25 records.
-Dell